Metrology method, target and substrate

ABSTRACT

A diffraction measurement target that has at least a first sub-target and at least a second sub-target, and wherein (1) the first and second sub-targets each include a pair of periodic structures and the first sub-target has a different design than the second sub-target, the different design including the first sub-target periodic structures having a different pitch, feature width, space width, and/or segmentation than the second sub-target periodic structure or (2) the first and second sub-targets respectively include a first and second periodic structure in a first layer, and a third periodic structure is located at least partly underneath the first periodic structure in a second layer under the first layer and there being no periodic structure underneath the second periodic structure in the second layer, and a fourth periodic structure is located at least partly underneath the second periodic structure in a third layer under the second layer.

This application is a continuation of U.S. patent application Ser. No.14/835,504, filed on Aug. 25, 2015, which claims priority under 35U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/090,801,filed on Dec. 11, 2014, and to U.S. Provisional Patent Application No.62/170,008, filed on Jun. 2, 2015, and priority under 35 U.S.C. § 119(a)to European Patent Application No. 14182962.2, filed on Aug. 29, 2014.The entire content of each of the foregoing applications is incorporatedherein in by reference.

FIELD

The present disclosure relates to a method, apparatus, and substrate formetrology usable, for example, in the manufacture of devices by alithographic technique and to a method of manufacturing devices using alithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

In order to monitor the lithographic process, one or more parameters ofthe patterned substrate are measured. Parameters may include, forexample, the overlay error between successive layers formed in or on thepatterned substrate and critical linewidth of developed photosensitiveresist. This measurement may be performed on a target surface of aproduct substrate and/or in the form of a dedicated metrology target.Metrology targets (or marks) may comprise, for example, combinations ofhorizontal and vertical bars, forming for example periodic structuressuch as gratings.

In a lithographic process (i.e., a process of developing a device orother structure involving lithographic exposure, which may typicallyinclude one or more associated processing steps such as development ofresist, etching, etc.), it is desirable frequently to make measurementsof structures created, e.g., for process control and verification.Various tools for making such measurements are known, including scanningelectron microscopes, which are often used to measure critical dimension(CD), and specialized tools to measure overlay, the accuracy ofalignment of two layers in a device. Recently, various forms ofscatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

SUMMARY

It is desirable to provide a method and apparatus for metrology using atarget, in which throughput, flexibility and/or accuracy can beimproved. Furthermore, although not limited to this, it would be ofgreat advantage, if this could be applied to small target structuresthat can be read out with a dark-field image-based technique.

In an embodiment, there is provided a method of measuring a parameter ofa lithographic process, the method comprising: illuminating adiffraction measurement target on a substrate with radiation, themeasurement target comprising at least a first sub-target and at least asecond sub-target, wherein the first and second sub-targets eachcomprise a pair of periodic structures and wherein the first sub-targethas a different design than the second sub-target, the different designcomprising the first sub-target periodic structures having a differentpitch, feature width, space width, and/or segmentation than the secondsub-target periodic structures; and detecting radiation scattered by atleast the first and second sub-targets to obtain for that target ameasurement representing the parameter of the lithographic process.

In an embodiment, there is provided a substrate having a diffractionmeasurement target, the measurement target comprising at least a firstsub-target and at least a second sub-target, wherein the first andsecond sub-targets each comprise a pair of periodic structures andwherein the first sub-target has a different design than the secondsub-target, the different design comprising the first sub-targetperiodic structures having a different pitch, feature width, spacewidth, and/or segmentation than the second sub-target periodicstructures.

In an embodiment, there is provided a method of measuring a parameter ofa lithographic process, the method comprising: illuminating adiffraction measurement target on a substrate with radiation, themeasurement target comprising at least a first sub-target and at least asecond sub-target in a first layer, wherein the first sub-targetcomprises a first periodic structure and the second sub-target comprisesa second periodic structure, wherein a third periodic structure islocated at least partly underneath the first periodic structure in asecond different layer under the first layer and there being no periodicstructure underneath the second periodic structure in the second layer,and wherein a fourth periodic structure is located at least partlyunderneath the second periodic structure in a third different layerunder the second layer; and detecting radiation scattered by at leastthe first through fourth periodic structures to obtain for that target ameasurement representing the parameter of the lithographic process.

In an embodiment, there is provided a substrate having a diffractionmeasurement target, the measurement target comprising at least a firstsub-target and at least a second sub-target, wherein the firstsub-target comprises a first periodic structure and the secondsub-target comprises a second periodic structure, wherein a thirdperiodic structure is located at least partly underneath the firstperiodic structure in a second different layer under the first layer andthere being no periodic structure underneath the second periodicstructure in the second layer, and wherein a fourth periodic structureis located at least partly underneath the second periodic structure in athird different layer under the second layer.

In an embodiment, there is provided a method of measuring a parameter ofa lithographic process, the method comprising: illuminating adiffraction measurement target on a substrate with radiation, themeasurement target comprising at least a first sub-target and at least asecond sub-target, wherein the first and second sub-targets eachcomprise a first pair of periodic structures having features extendingin a first direction and a second pair of periodic structures havingfeatures extending in a second different direction, and wherein thefirst sub-target has a different design than the second sub-target; anddetecting radiation scattered by at least the first and secondsub-targets to obtain for that target a measurement representing theparameter of the lithographic process.

In an embodiment, there is provided a substrate having a diffractionmeasurement target, the measurement target comprising at least a firstsub-target and at least a second sub-target, wherein the first andsecond sub-targets each comprise a first pair of periodic structureshaving features extending in a first direction and a second pair ofperiodic structures having features extending in a second differentdirection, and wherein the first sub-target has a different design thanthe second sub-target.

In an embodiment, there is provided a method of measuring a parameter ofa lithographic process, the method comprising: illuminating adiffraction measurement target on a substrate with radiation, themeasurement target comprising at least a first sub-target and at least asecond sub-target, wherein the first and second sub-targets eachcomprise a first pair of periodic structures having features extendingin a first direction and a second pair of periodic structures havingfeatures extending in a second different direction, and wherein at leastpart of each of the periodic structures of the first and secondsub-targets is within a contiguous area of less than or equal to 1000μm² on the substrate; and detecting radiation scattered by at least thefirst and second sub-targets to obtain for that target a measurementrepresenting the parameter of the lithographic process.

In an embodiment, there is provided a substrate having a diffractionmeasurement target, the measurement target comprising at least a firstsub-target and at least a second sub-target, wherein the first andsecond sub-targets each comprise a first pair of periodic structureshaving features extending in a first direction and a second pair ofperiodic structures having features extending in a second differentdirection, and wherein at least part of each of the periodic structuresof the first and second sub-targets is within a contiguous area of lessthan or equal to 1000 μm² on the substrate.

In an embodiment, there is provided a method of metrology target design,the method comprising: receiving an indication for the design of adiffractive metrology target having a plurality of sub-targets, eachsub-target comprising a first pair of periodic structures havingfeatures extending in a first direction and a second pair of periodicstructures having features extending in a second different direction;receiving a constraint on the area, a dimension, or both, of thediffractive metrology target; and selecting, by a processor, a design ofthe diffractive metrology target based at least on the constraint.

In an embodiment, there is provided a diffraction measurement targetcomprising at least a first sub-target and at least a second sub-target,wherein the first and second sub-targets each comprise a pair ofperiodic structures and wherein the first sub-target has a differentdesign than the second sub-target, the different design comprising thefirst sub-target periodic structures having a different pitch, featurewidth, space width, and/or segmentation than the second sub-targetperiodic structures.

In an embodiment, there is provided a diffraction measurement targetcomprising at least a first sub-target and at least a second sub-targetthat, when on a substrate, are in a first layer, wherein the firstsub-target comprises a first periodic structure and the secondsub-target comprises a second periodic structure, and comprising a thirdperiodic structure, when on the substrate, located at least partlyunderneath the first periodic structure in a second different layerunder the first layer and there being no periodic structure underneaththe second periodic structure in the second layer, and comprising afourth periodic structure, when on the substrate, located at leastpartly underneath the second periodic structure in a third differentlayer under the second layer.

In an embodiment, there is provided a diffraction measurement targetcomprising at least a first sub-target and at least a second sub-target,wherein the first and second sub-targets each comprise a first pair ofperiodic structures having features extending in a first direction and asecond pair of periodic structures having features extending in a seconddifferent direction, and wherein the first sub-target has a differentdesign than the second sub-target.

In an embodiment, there is provided a diffraction measurement targetcomprising at least a first sub-target and at least a second sub-target,wherein the first and second sub-targets each comprise a first pair ofperiodic structures having features extending in a first direction and asecond pair of periodic structures having features extending in a seconddifferent direction, and wherein at least part of each of the periodicstructures of the first and second sub-targets is within a contiguousarea of less than or equal to 1000 μm² on a substrate.

In an embodiment, there is provided a method comprising: illuminatingwith radiation a diffraction measurement target on a substrate, themeasurement target comprising at least a first sub-target, a secondsub-target and a third sub-target, wherein the first, second and thirdsub-targets are different in design.

In an embodiment, there is provided a diffraction metrology targetcomprising at least a first sub-target, a second sub-target and a thirdsub-target, wherein the first, second and third sub-targets aredifferent in design.

In an embodiment, there is provided a method comprising measuringoverlay between two layers, the method comprising: illuminating withradiation a diffraction measurement target on a substrate having aportion of the target on each of the two layers, wherein the two layersare separated by at least one other layer.

In an embodiment, there is provided a method of devising a measurementtarget, having a plurality of sub-targets, involving locating an assistfeature at a periphery of the sub-targets, the assist feature beingconfigured to reduce measured intensity peaks at the periphery of thesub-targets. Thus, in an embodiment, there is provided a method ofdevising an arrangement of a diffraction measurement target, the targetcomprising a plurality of sub-targets, each sub-target comprising aplurality of periodic structures and each sub-target designed to measurea different layer-pair or to measure for a different process stack, themethod comprising: locating the periodic structures of the sub-targetswithin a target area; and locating an assist feature at a periphery ofthe sub-targets, the assist feature configured to reduce a measuredintensity peak at the periphery of the sub-targets.

In an embodiment, there is provided a diffraction measurement targetcomprising: a plurality of sub-targets in a target area of the target,each sub-target comprising a plurality of periodic structures and eachsub-target designed to measure a different layer-pair or to measure fora different process stack; and an assist feature at the periphery of thesub-targets, the assist feature configured to reduce a measuredintensity peak at the periphery of the sub-targets.

In an embodiment, there is provided a method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including inspecting at least adiffraction measurement target formed as part of or beside the devicepattern on at least one of the substrates using a method as describedherein and controlling the lithographic process for later substrates inaccordance with the result of the method.

In an embodiment, there is provided a patterning device configured to atleast in part form a diffraction measurement target as described herein.

In an embodiment, there is provided a non-transitory computer programproduct comprising machine-readable instructions for causing a processorto cause performance of a method as described herein.

In an embodiment, there is provided a non-transitory computer programproduct comprising machine-readable instructions for causing a processorto cause performance of a method as described herein.

In an embodiment, there is provided a non-transitory computer programproduct comprising machine-readable instructions or data defining atarget as described herein.

In an embodiment, there is provided a substrate comprising a target asdescribed herein.

In an embodiment, there is provided a system comprising: an inspectionapparatus configured to provide a beam on a diffraction measurementtarget on a substrate and to detect radiation diffracted by the targetto determine a parameter of a lithographic process; and a non-transitorycomputer program product as described herein.

Features and/or advantages of embodiments of the invention, as well asthe structure and operation of various embodiments of the invention, aredescribed in detail herein with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which.

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIG. 3(a) is schematic diagram of a dark field scatterometer for use inmeasuring targets according to embodiments of the invention using afirst pair of illumination apertures providing certain illuminationmodes;

FIG. 3(b) is a schematic detail of a diffraction spectrum of a targetperiodic structure for a given direction of illumination;

FIG. 3(c) is a schematic illustration of a second pair of illuminationapertures providing further illumination modes in using a scatterometerfor diffraction based overlay measurements;

FIG. 3(d) is a schematic illustration of a third pair of illuminationapertures combining the first and second pairs of apertures providingfurther illumination modes in using a scatterometer for diffractionbased overlay measurements;

FIG. 4 depicts a form of multiple periodic structure (e.g., grating)target and an outline of a measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in theapparatus of FIG. 3;

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the apparatus of FIG. 3 and adaptable to embodiments of thepresent invention;

FIGS. 7(a) to 7(c) show schematic cross-sections of overlay periodicstructures having different overlay values in the region of zero;

FIG. 8 illustrates principles of overlay measurement in an ideal targetstructure;

FIG. 9 illustrates an extended operating range metrology targetaccording to an embodiment of the invention;

FIG. 10 illustrates use of an extended operating range metrology targetaccording to an embodiment of the invention to account for process stackvariation;

FIG. 11 illustrates use of an extended operating range metrology targetaccording to an embodiment of the invention for multiple layer overlaymeasurement;

FIGS. 12A-E illustrate variations of an extended operating rangemetrology target according to an embodiment of the invention;

FIG. 13(a) depicts an example of a non-optimized target layout;

FIG. 13(b) depicts a resulting dark field image of the target layout ofFIG. 13(a);

FIGS. 14(a) to (f) illustrate examples of a non-optimized target layoutand a target layout according to an embodiment of the invention, and ofexpected resulting dark field images of these targets using differentmeasurement radiation wavelengths;

FIG. 15 illustrates a partial cross section of a target according to anembodiment of the invention;

FIG. 16(a) illustrates an example of a non-optimized target layout;

FIG. 16(b) illustrates an example of a target layout according to anembodiment of the invention;

FIG. 17 is a flowchart of a method of devising a target arrangementaccording to an embodiment of the invention;

FIGS. 18(a)-(f) illustrate an embodiment of the method depicted in FIG.17 being performed to devise a target arrangement;

FIG. 19 schematically depicts a system to design an extended operatingrange metrology target according to an embodiment of the invention;

FIG. 20 depicts a flowchart illustrating a process of designing anextended operating range metrology target according to an embodiment ofthe invention;

FIG. 21 depicts a flowchart illustrating a process in which the extendedoperating range metrology target is used to monitor performance, and asa basis for controlling metrology, design and/or production processesaccording to an embodiment of the invention;

FIGS. 22(A)-(C) illustrate an extended operating range metrology targetaccording to an embodiment of the invention;

FIGS. 23(A)-(C) illustrate an extended operating range metrology targetaccording to an embodiment of the invention;

FIGS. 24(A)-(C) illustrate an extended operating range metrology targetaccording to an embodiment of the invention;

FIGS. 25(A)-(C) illustrate an extended operating range metrology targetaccording to an embodiment of the invention; and

FIGS. 26(A)-(E) illustrate an extended operating range metrology targetaccording to an embodiment of the invention

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments in detail, it is instructive to present anexample environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; a substrate table (e.g., awafer table) WT constructed to hold a substrate (e.g., a resist coatedwafer) W and connected to a second positioner PW configured toaccurately position the substrate in accordance with certain parameters;and a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M₁, M₂ and substrate alignment marks P₁, P₂.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. An embodiment of an alignmentsystem, which can detect the alignment markers, is described furtherbelow.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WTa is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WTa relative to the patterning device support (e.g.,mask table) MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WTa is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WTa or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which has atleast two tables WTa, WTb (e.g., two substrate tables) and at least twostations—an exposure station and a measurement station—between which atleast one of the tables can be exchanged. For example, while a substrateon one table is being exposed at the exposure station, another substratecan be loaded onto the other substrate table at the measurement stationand various preparatory steps carried out. The preparatory steps mayinclude mapping the surface control of the substrate using a levelsensor LS and measuring the position of alignment markers on thesubstrate using an alignment sensor AS, both sensors being supported bya reference frame RF. If the position sensor IF is not capable ofmeasuring the position of a table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the table to be tracked at bothstations. As another example, while a substrate on one table is beingexposed at the exposure station, another table without a substrate waitsat the measurement station (where optionally measurement activity mayoccur). This other table has one or more measurement devices and mayoptionally have other tools (e.g., cleaning apparatus). When thesubstrate has completed exposure, the table without a substrate moves tothe exposure station to perform, e.g., measurements and the table withthe substrate moves to a location (e.g., the measurement station) wherethe substrate is unloaded and another substrate is load. Thesemulti-table arrangements enable a substantial increase in the throughputof the apparatus.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to as a lithocell orlithocluster, which also includes apparatus to perform one or more pre-and post-exposure processes on a substrate. Conventionally these includeone or more spin coaters SC to deposit a resist layer, one or moredevelopers DE to develop exposed resist, one or more chill plates CH andone or more bake plates BK. A substrate handler, or robot, RO picks up asubstrate from input/output ports I/O1, I/O2, moves it between thedifferent process devices and delivers it to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithographic controlunit LACU. Thus, the different apparatus may be operated to maximizethroughput and processing efficiency.

In order that the substrate that is exposed by the lithographicapparatus is exposed correctly and consistently, it is desirable toinspect an exposed substrate to measure one or more properties such asoverlay error between subsequent layers, line thickness, criticaldimension (CD), etc. If an error is detected, an adjustment may be madeto an exposure of one or more subsequent substrates, especially if theinspection can be done soon and fast enough that another substrate ofthe same lot/batch is still to be exposed. Also, an already exposedsubstrate may be stripped and reworked (to improve yield) or discarded,thereby avoiding performing an exposure on a substrate that is known tobe faulty. In a case where only some target portions of a substrate arefaulty, a further exposure may be performed only on those targetportions which are good. Another possibility is to adapt a setting of asubsequent process step to compensate for the error, e.g. the time of atrim etch step can be adjusted to compensate for substrate-to-substrateCD variation resulting from the lithographic process step.

An inspection apparatus is used to determine one or more properties of asubstrate, and in particular, how one or more properties of differentsubstrates or different layers of the same substrate vary from layer tolayer and/or across a substrate. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measures one or more propertiesin the exposed resist layer immediately after the exposure. However, thelatent image in the resist has a very low contrast—there is only a verysmall difference in refractive index between the part of the resistwhich has been exposed to radiation and that which has not—and not allinspection apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on an exposed substrate and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibility for rework of a faulty substrate butmay still provide useful information, e.g. for the purpose of processcontrol.

A target used by a conventional scatterometer comprises a relativelylarge periodic structure (e.g., grating) layout, e.g., 40 μm by 40 μm.In that case, the measurement beam often has a spot size that is smallerthan the periodic structure layout (i.e., the periodic structure layoutis underfilled). This simplifies mathematical reconstruction of thetarget as it can be regarded as infinite. However, for example, so thetarget can be positioned in among product features, rather than in thescribe lane, the size of a target has been reduced, e.g., to 20 μm by 20μm or less, or to 10 μm by 10 μm or less. In this situation, theperiodic structure layout may be made smaller than the measurement spot(i.e., the periodic structure layout is overfilled). Typically such atarget is measured using dark field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Examples of dark fieldmetrology can be found in PCT patent application publication nos. WO2009/078708 and WO 2009/106279, which are hereby incorporated in theirentirety by reference. Further developments of the technique have beendescribed in U.S. patent application publications US2011-0027704,US2011-0043791 and US2012-0242970, which are hereby incorporated intheir entirety by reference. Diffraction-based overlay using dark-fielddetection of the diffraction orders enables overlay measurements onsmaller targets. These targets can be smaller than the illumination spotand may be surrounded by product structures on a substrate. In anembodiment, multiple targets can be measured in one image.

A dark field metrology apparatus suitable for use in embodiments of theinvention is shown in FIG. 3(a). A target T (comprising a periodicstructure) and diffracted rays are illustrated in more detail in FIG.3(b). The dark field metrology apparatus may be a stand-alone device orincorporated in either the lithographic apparatus LA, e.g., at themeasurement station, or the lithographic cell LC. An optical axis, whichhas several branches throughout the apparatus, is represented by adotted line O. In this apparatus, radiation emitted by an output 11(e.g., a source such as a laser or a xenon lamp or an opening connectedto a source) is directed onto substrate W via a prism 15 by an opticalsystem comprising lenses 12, 14 and objective lens 16. These lenses arearranged in a double sequence of a 4F arrangement. A different lensarrangement can be used, provided that it still provides a substrateimage onto a detector.

In an embodiment, the lens arrangement allows for access of anintermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can beselected by defining a spatial intensity distribution in a plane thatpresents the spatial spectrum of the substrate plane, here referred toas a (conjugate) pupil plane. In particular, this can be done, forexample, by inserting an aperture plate 13 of suitable form betweenlenses 12 and 14, in a plane which is a back-projected image of theobjective lens pupil plane. In the example illustrated, aperture plate13 has different forms, labeled 13N and 13S, allowing differentillumination modes to be selected. The illumination system in thepresent examples forms an off-axis illumination mode. In the firstillumination mode, aperture plate 13N provides off-axis illuminationfrom a direction designated, for the sake of description only, as‘north’. In a second illumination mode, aperture plate 13S is used toprovide similar illumination, but from a different (e.g., opposite)direction, labeled ‘south’. Other modes of illumination are possible byusing different apertures. The rest of the pupil plane is desirably darkas any unnecessary radiation outside the desired illumination mode mayinterfere with the desired measurement signals.

As shown in FIG. 3(b), target T is placed with substrate W substantiallynormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on target T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). With an overfilled small target T,these rays are just one of many parallel rays covering the area of thesubstrate including metrology target T and other features. Where acomposite periodic structure target is provided, each individualperiodic structure within the target will give rise to its owndiffraction spectrum. Since the aperture in plate 13 has a finite width(necessary to admit a useful quantity of radiation), the incident rays Iwill in fact occupy a range of angles, and the diffracted rays 0 and+1/−1 will be spread out somewhat. According to the point spreadfunction of a small target, each order +1 and −1 will be further spreadover a range of angles, not a single ideal ray as shown. Note that theperiodic structure pitch and illumination angle can be designed oradjusted so that the first order rays entering the objective lens areclosely aligned with the central optical axis. The rays illustrated inFIGS. 3(a) and 3(b) are shown somewhat off axis, purely to enable themto be more easily distinguished in the diagram.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through prism 15.Returning to FIG. 3(a), both the first and second illumination modes areillustrated, by designating diametrically opposite (in this case)apertures labeled as north (N) and south (S). When the incident ray I isfrom the north side of the optical axis, that is when the firstillumination mode is applied using aperture plate 13N, the +1 diffractedrays, which are labeled +1(N), enter the objective lens 16. In contrast,when the second illumination mode is applied using aperture plate 13Sthe −1 diffracted rays (labeled −1(S)) are the ones which enter the lens16. Thus, in an embodiment, measurement results are obtained bymeasuring the target twice under certain conditions, e.g., afterrotating the target or changing the illumination mode or changing theimaging mode to obtain separately the −1^(st) and the +1^(st)diffraction order intensities. Comparing these intensities for a giventarget provides a measurement of asymmetry in the target, and asymmetryin the target can be used as an indicator of a parameter of alithography process, e.g., overlay error. In the situation describedabove, the illumination mode is changed.

A beam splitter 17 divides the diffracted beams into two measurementbranches. In a first measurement branch, optical system 18 forms adiffraction spectrum (pupil plane image) of the target on first sensor19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for forasymmetry measurement as well as for many measurement purposes such asreconstruction, which are not described in detail here. The firstexamples to be described will use the second measurement branch tomeasure asymmetry.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g. a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageDF of the target formed on sensor 23 is formed from the −1 or +1 firstorder beam. The images captured by sensors 19 and 23 are output to imageprocessor and controller PU, the function of which will depend on theparticular type of measurements being performed. Note that the term‘image’ is used here in a broad sense. An image of the features of aperiodic structure of the target as such will not be formed, if only oneof the −1 and +1 orders is present.

The particular forms of aperture plate 13 and stop 21 shown in FIG. 3are purely examples. In another embodiment, on-axis illumination of thetargets is used and an aperture stop with an off-axis aperture is usedto pass substantially only one first order of diffracted radiation tothe sensor (the apertures shown at 13 and 21 are effectively swapped inthat case). In yet other embodiments, 2nd, 3rd and higher order beams(not shown in FIG. 3) can be used in measurements, instead of or inaddition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Alternatively or in addition, a set of plates 13 could beprovided and swapped, to achieve the same effect. A programmableillumination device such as a deformable mirror array or transmissivespatial light modulator can be used also. Moving mirrors or prisms canbe used as another way to adjust the illumination mode.

As just explained in relation to aperture plate 13, the selection ofdiffraction orders for imaging can alternatively be achieved by alteringthe aperture stop 21, or by substituting a pupil-stop having a differentpattern, or by replacing the fixed field stop with a programmablespatial light modulator. In that case the illumination side of themeasurement optical system can remain constant, while it is the imagingside that has first and second modes. In practice, there are manypossible types of measurement method, each with its own advantages anddisadvantages. In one method, the illumination mode is changed tomeasure the different orders. In another method, the imaging mode ischanged. In a third method, the illumination and imaging modes remainunchanged, but the target is rotated through, e.g., 180 degrees. In eachcase the desired effect is the same, namely to select first and secondportions of the non-zero order diffracted radiation which are, e.g.,symmetrically opposite one another in the diffraction spectrum of thetarget.

While the optical system used for imaging in the present examples has awide entrance pupil which is restricted by the aperture stop 21, inother embodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop. Different aperture plates are shownin FIGS. 3(c) and (d) which can be used as described further below.

Typically, a target will be aligned with its periodic structure featuresrunning either north-south or east-west. That is to say, a periodicstructure (e.g., grating) will be aligned in the X direction or the Ydirection of the substrate W. But, it may be angled at a differentangle, i.e., at 45°. Aperture plate 13N or 13S is used to measure aperiodic structure of a target oriented in one direction (e.g., X, Y orother direction depending on the set-up). For measurement of a periodicstructure at another angle (e.g., substantially orthogonal), rotation ofthe target might be implemented (e.g., rotation through 90° and 270° forsubstantially orthogonal periodic structures). Or, illumination fromanother angle (e.g., east or west) may be provided in the illuminationoptics, using the aperture plate 13E or 13W, shown in FIG. 3(c), whichmay have the apertures at the appropriate angle (e.g., east or west).The aperture plates 13N to 13W can be separately formed andinterchanged, or they may be a single aperture plate which can berotated by appropriate angle (e.g., 90, 180 or 270 degrees).

Different aperture plates are shown in FIGS. 3(c) and (d). FIG. 3(c)illustrates two further types of off-axis illumination mode. In a firstillumination mode of FIG. 3(c), aperture plate 13E provides off-axisillumination from a direction designated, for the sake of descriptiononly, as ‘east’ relative to the ‘north’ previously described. As notedabove, the ‘east’ may be at a different angle than as shown. In a secondillumination mode of FIG. 3(c), aperture plate 13W is used to providesimilar illumination, but from a different (e.g., opposite) direction,labeled ‘west’. FIG. 3(d) illustrates two further types of off-axisillumination mode. In a first illumination mode of FIG. 3(d), apertureplate 13NW provides off-axis illumination from the directions designated‘north’ and ‘west’ as previously described. In a second illuminationmode, aperture plate 13SE is used to provide similar illumination, butfrom a different (e.g., opposite) direction, labeled ‘south’ and ‘east’as previously described. Provided that crosstalk between these differentdiffraction signals is not too great, measurements of periodicstructures extending in different directions (e.g., both X and Y) can beperformed without changing the illumination mode. The use of these, andnumerous other variations and applications of the apparatus aredescribed in, for example, the prior published patent applicationpublications mentioned above. As mentioned already, the off-axisapertures illustrated in FIGS. 3(c) and (d) could be provided in theaperture stop 21 instead of in aperture plate 13. In that case, theillumination would be on axis.

FIG. 4 depicts an example composite metrology target formed on asubstrate. The composite target comprises four periodic structures(e.g., gratings) 32, 33, 34, 35 positioned closely together. In anembodiment, the periodic structures are positioned closely togetherenough so that they all are within a measurement spot 31 formed by theillumination beam of the metrology apparatus. In that case, the fourperiodic structures thus are all simultaneously illuminated andsimultaneously imaged on sensors 19 and 23. In an example dedicated tooverlay measurement, periodic structures 32, 33, 34, 35 are themselvescomposite periodic structures formed by overlying periodic structures ofanother target that is patterned in a different layer of the deviceformed on substrate W. Such a target may have outer dimensions within 20μm×20 μm or within 16 μm×16 μm. Further, all the periodic structures areused to measure overlay between a particular pair of layers. Tofacilitate a target being able to measure more than a single pair oflayers, periodic structures 32, 33, 34, 35 may have differently biasedoverlay offsets in order to facilitate measurement of overlay betweendifferent layers in which the different parts of the composite periodicstructures are formed. Thus, all the periodic structures for the targeton the substrate would be used to measure one pair of layers and all theperiodic structures for another same target on the substrate would beused to measure another pair of layers, wherein the overlay biasfacilitates distinguishing between the layer-pairs. The meaning ofoverlay bias will be explained below, particularly with reference toFIG. 7.

FIGS. 7(a)-(c) show schematic cross sections of overlay periodicstructures of respective targets T, with different biases. These can beused on substrate W, as seen in FIGS. 3 and 4. Periodic structures withperiodicity in the X direction are shown for the sake of example only.Different combinations of these periodic structures with differentbiases and with different orientations can be provided.

Starting with FIG. 7(a), a composite overlay target 600 formed in twolayers, labeled L1 and L2, is depicted. In the lower layer L1, a firstperiodic structure is formed by features (e.g., lines) 602 and spaces604 on a substrate 606. In layer L2, a second periodic structure isformed by features (e.g., lines) 608 and spaces 610. (The cross-sectionis drawn such that the features 602, 608 extend into the page.) Theperiodic structure pattern repeats with a pitch P in both layers. Lines602 and 608 are mentioned for the sake of example only, other types offeatures such as dots, blocks and via holes can be used. In thesituation shown at FIG. 7(a), there is no overlay error and no bias, sothat each periodic structure feature 608 lies exactly over a periodicstructure feature 602 in the lower periodic structure.

At FIG. 7(b), the same target with a bias +d is depicted such that thefeatures 608 of the upper periodic structure are shifted by a distance dto the right, relative to the features 602 of the lower periodicstructure. That is, features 608 and features 602 are arranged so thatif they were both printed exactly at their nominal locations, features608 would be offset relative to the features 602 by the distance d. Thebias distance d might be a few nanometers in practice, for example 5-60nm, while the pitch P is for example in the range 300-1000 nm, forexample 500 nm or 600 nm. At FIG. 7(c), the same target with a bias −dis depicted such that the features 608 are shifted to the left relativeto the features 602. Biased targets of this type shown at FIGS. 7(a) to(c) are described in, for example, the patent application publicationsmentioned above.

Further, while FIGS. 7(a)-(c) depicts the features 608 lying over thefeatures 602 (with or without a small bias of +d or −d applied), whichis referred to as a “line on line” target having a bias in the region ofzero, a target may have a programmed bias of P/2, that is half thepitch, such that each feature 608 in the upper periodic structure liesover a space 604 in the lower periodic structure. This is referred to asa “line on trench” target. In this case, a small bias of +d or −d mayalso be applied. The choice between “line on line” target or a “line ontrench” target depends on the application.

Returning to FIG. 4, periodic structures 32, 33, 34, 35 may also differin their orientation, as shown, so as to diffract incoming radiation inX and Y directions. In one example, periodic structures 32 and 34 areX-direction periodic structures with biases of +d, −d, respectively.Periodic structures 33 and 35 may be Y-direction periodic structureswith offsets +d and −d respectively. While four periodic structures areillustrated, another embodiment may include a larger matrix to obtaindesired accuracy. For example, a 3×3 array of nine composite periodicstructures may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d, +4d.Separate images of these periodic structures can be identified in theimage captured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3(d). While the sensor19 cannot resolve the different individual periodic structures 32 to 35,the sensor 23 can do so. The cross-hatched rectangle represents thefield of the image on the sensor, within which the illuminated spot 31on the substrate is imaged into a corresponding circular area 41. In anembodiment, the field is dark. Within this image, rectangular areas42-45 represent the images of the periodic structures 32 to 35. If theperiodic structures are located in product areas, product features mayalso be visible in the periphery of this image field. While only asingle composite grating target is shown in the dark field image of FIG.5, in practice a product made by lithography may have many layers, andoverlay measurements are desired to be made between different pairs oflayers. For each overlay measurement between pair of layers, one or morecomposite grating targets are used, and therefore other compositetargets may be present within the image field. Image processor andcontroller PU processes these images using pattern recognition toidentify the separate images 42 to 45 of periodic structures 32 to 35.In this way, the images do not have to be aligned very precisely at aspecific location within the sensor frame, which greatly improvesthroughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures have beenidentified, the intensities of those individual images can be measured,e.g., by averaging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan example of such a parameter. For example, comparing the intensitiesreveals asymmetries that can be used as a measure of overlay. In anothertechnique for measuring asymmetry and hence overlay, the sensor 19 isused.

FIG. 6 illustrates how, using for example the method described in PCTpatent application publication no. WO 2011/012624 and U.S. patentapplication publication no. 2011/027704 and using for example theapparatus of FIGS. 3 and 4, overlay error between the two layerscontaining the component periodic structures 32 to 35 is measuredthrough asymmetry of the periodic structures, as revealed by comparingtheir intensities in the +1 order and −1 order dark field images.

At step M1, the substrate, for example a semiconductor wafer, isprocessed through the lithographic cell of FIG. 2 one or more times, tocreate a structure including the target comprising periodic structures32-35 that form a metrology target. At M2, using the metrology apparatusof FIG. 3, an image of the periodic structures 32 to 35 is obtainedusing one of the first order diffracted beams (say −1). In anembodiment, a first illumination mode (e.g., the illumination modecreated using aperture plate 13NW) is used. Then, whether by changingthe illumination mode, or changing the imaging mode, or by rotatingsubstrate W by 180° in the field of view of the metrology apparatus, asecond image of the periodic structures using the other first orderdiffracted beam (+1) can be obtained (step M3). Consequently, the +1diffracted radiation is captured in the second image. In an embodiment,the illuminated mode is changed and a second illumination mode (e.g.,the illumination mode created using aperture plate 13SE) is used. It isa matter of design choice whether all the periodic structures can becaptured in each image, or whether there needs to be relative movementbetween the measurement apparatus and the substrate so as to capture theperiodic structures in separate images. In either case, it is assumedthat first and second images of all the component periodic structuresare captured via sensor 23.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual periodicstructure features are not resolved, because only one of the +1 and −1order diffracted radiation is present. Each periodic structure will berepresented simply by an area of a certain intensity level. In step M4,a region of interest (ROI) is identified within the image of eachcomponent periodic structure, from which intensity levels will bemeasured. This is done because, particularly around the edges of theindividual grating images, intensity values can be highly dependent onprocess variables such as resist thickness, composition, line shape, aswell as edge effects generally.

Having identified the region of interest P1, P2, P3, P4 for eachrespective individual periodic structure 32-35 and measured itsintensity, the asymmetry of the periodic structure, and hence, e.g.,overlay error, can then be determined. This is done by the imageprocessor and controller PU in step M5 comparing the intensity valuesobtained for +1 and −1 orders for each periodic structure 32-35 toidentify any difference in their intensity, i.e., an asymmetry. The term“difference” is not intended to refer only to subtraction. Differencesmay be calculated in ratio form. Thus, the intensity difference iscalculated at step M5 to obtain a measurement of asymmetry for eachperiodic structure. In step M6 the measured asymmetries for a number ofperiodic structures are used together with, if applicable, knowledge ofthe overlay biases of those periodic structures to calculate one or moreperformance parameters of the lithographic process in the vicinity ofthe target T. A performance parameter of interest may be overlay. Otherparameters of performance of the lithographic process can be calculatedsuch as focus and/or dose. The one or more performance parameters can befed back for improvement of the lithographic process, and/or used toimprove the measurement and calculation process of FIG. 6 itself.

In an embodiment to determine overlay, FIG. 8 depicts a curve 702 thatillustrates the relationship between overlay error OV and measuredasymmetry A for an ‘ideal’ target having zero offset and no featureasymmetry within the individual periodic structures forming the overlayperiodic structure. This graph is to illustrate the principles ofdetermining the overlay only, and in the graph, the units of measuredasymmetry A and overlay error OV are arbitrary.

In the ‘ideal’ situation of FIGS. 7(a)-(c), the curve 702 indicates thatthe measured asymmetry A has a sinusoidal relationship with the overlay.The period P of the sinusoidal variation corresponds to the period(pitch) of the periodic structures, converted of course to anappropriate scale. The sinusoidal form is pure in this example, but caninclude harmonics in real circumstances. For the sake of simplicity, itis assumed in this example (a) that only first order diffractedradiation from the target reaches the image sensor 23 (or its equivalentin a given embodiment), and (b) that the experimental target design issuch that within these first orders a pure sine-relation exists betweenintensity and overlay between top and lower periodic structures results.Whether this is true in practice is a function of the optical systemdesign, the wavelength of the illuminating radiation and the pitch P ofthe periodic structure, and the design and stack of the target.

As mentioned above, biased periodic structures can be used to measureoverlay, rather than relying on a single measurement. This bias has aknown value defined in the patterning device (e.g. a reticle) from whichit was made, that serves as an on-substrate calibration of the overlaycorresponding to the measured signal. In the drawing, the calculation isillustrated graphically. In steps M1-M5 of FIG. 6, asymmetrymeasurements A(+d) and A(−d) are obtained for component periodicstructures having biases +d and −d respectively (as shown in FIGS. 7(b)and 7(c), for example). Fitting these measurements to the sinusoidalcurve gives points 704 and 706 as shown. Knowing the biases, the trueoverlay error OV can be calculated. The pitch P of the sinusoidal curveis known from the design of the target. The vertical scale of the curve702 is not known to start with, but is an unknown factor which we cancall a 1^(st) harmonic proportionality constant, K₁.

In equation terms, the relationship between overlay and asymmetry isassumed to be:

A=K ₁·sin(OV)

where OV is expressed on a scale such that the periodic structure pitchP corresponds to an angle 2π radians. Using two measurements withperiodic structures with different, known biases one can solve twoequations to calculate the unknowns K₁ and overlay OV.

The metrology target described above is designed for one or moreparticular layers associated with a particular process stack (i.e., theprocess stack being the processes and material used to construct aparticular device or part thereof for the layer, e.g., the one ormaterial layers involved (e.g., the thickness and/or material typethereof), the lithographic exposure process, the resist developmentprocess, the bake process, the etch process, etc.) with the flexibilitythat the metrology target will provide measurement robustness fornominal changes in the process stack. That is, the metrology target isdesigned using knowledge of the process layers (e.g., their material,thickness, etc.), the processing steps applied to the layers, etc. toarrive at a metrology target that will give good, if not optimal,measurement results for the parameter of the lithographic process beingmeasured.

However, during lithographic process development, the process stack fora certain layer can change significantly beyond the nominal. An existingtarget cannot handle a large change in the process stack (i.e., aprocess change). Thus, multiple targets may be designed to aim forextremes of such changes. This requires a new target design, which meansthe process development has to wait for a significant period of timebefore such a new target is, for example, taped-out on the mask; thus,R&D cycle time is increased significantly. Moreover, multiple targetscan mean significant costs in creating different patterning devices(e.g., masks) for each different target. Or, the space to accommodatesuch targets (i.e., available space on the patterning device pattern)may not be available and/or the throughput to measure such multipletargets can be significantly impacted.

Further, a typical diffraction-based overlay target is used to measureoverlay between a pair of layers. But, new processes (e.g.,multi-patterning processes, via-last processes, etc.) are driving a needto do overlay measurements between not only a single layer-pair butamong multiple layer-pairs. Similarly to the process development examplediscussed above, a solution for multi-layer overlay would be to increasethe number of overlay targets (i.e., different targets needed fordifferent layer-pairs) and hence the number of measurements increase(i.e., a measurement for each pair of the multi-layer combinations).This is at a cost of target “real estate” (i.e., available space on thepatterning device pattern to accommodate these individual layer-pairtargets) and throughput due to the increased measurement times.

So, according to an embodiment of the invention, there is provided adiffraction metrology target comprising a multi-periodic structuretarget-cluster (a single cluster of periodic structures) that is smallin total size, but includes a set of multi-design periodic structures;for convenience of reference, this target is referred to as an extendedoperating range metrology target. So, for, e.g., process development, asub-set of periodic structures from the extended operating rangemetrology target can be used for a certain process stack condition whileanother sub-set(s) of periodic structures from the extended operatingrange metrology target can be used for another process stack conditionthus being able to account for significant variations in the processstack. Alternatively or additionally, for, e.g., multi-layer overlay, asub-set of periodic structures from the extended operating rangemetrology target can be used for a certain layer-pair while anothersub-set(s) of periodic structures of the extended operating rangemetrology target can be used for another layer-pair thus enablingmulti-layer overlay.

Thus, in the situation of significant process stack variation (e.g.,variation of the process stack that can't be properly handled by aparticular periodic structure design of a metrology target), theextended operating range metrology target allows putting significantlydifferent designs (all within a reasonable size of a target) that willincrease the chance of successful measurement results if a change ismade to the process stack. This could increase the chance of first timemeasurement success due the presence of different designs pro-activelyanticipating for process stack variations. And, in the situation ofmulti-overlay measurement, the extended operating range metrology targetallows measuring of overlay between multiple layers in one measurementsequence. That is, in an embodiment, multiple layer-pairs can bemeasured in one measurement sequence and in an embodiment, thediffraction data of multiple layer-pairs can be detected simultaneously.

By having the differently designed periodic structures in the extendedoperating range metrology target, significant variations in the processstack and/or multi-layer can be handled by a single metrology targetwith differently designed sets of periodic structures therein. Thereby,the cost of creating different patterning devices (e.g., masks) for eachdifferent individual target and/or the cost of measuring time can besignificantly reduced. Further, by the relatively small size of theextended operating range metrology target, the cost of target “realestate” (i.e., available space on the patterning device pattern toaccommodate these individual layer-pair targets) for multiple differentindividual targets and the cost of throughput due to the increasedmeasurement times may be significantly reduced. So, the extendedoperating range metrology target can bring all these multiple targetswithin a single target-cluster that is small enough from a real-estatepoint of view and also more favorable in terms of measurement timecompared to multiple individual targets.

Referring to FIG. 9, an embodiment of an extended operating rangemetrology target 800 is depicted. The extended operating range metrologytarget 800 comprises a plurality of sub-targets, in this example, fourdiffraction sub-targets 802, 804, 806, 808. As will be appreciated, adifferent number of sub-targets may be provided. For example, just twosub-targets may be provided. Alternatively, three, five, six, seven,eight, etc. sub-targets may be provided. In an embodiment, eachsub-target 802-808 is separated from a neighboring sub-target by a gap820. In an embodiment, the gap is 200 nm or more, 250 nm or more, 350 nmor more, 500 nm or more, 750 nm or more, or 1 μm or more. The gapfacilitates reconstruction of the sub-targets so that they can beseparately identified. Further, the gap may help prevent cross-talk ofdiffraction from one sub-target extending over to another sub-target.

Each sub-target comprises a periodic structure. In an embodiment, eachsub-target comprises at least a pair of periodic structures. In anembodiment, each sub-target comprises at least two pairs of periodicstructures. In an embodiment, the features (e.g., lines) of the periodicstructures in a sub-target extend in a same direction. In an embodiment,at least one periodic structure of a sub-target may have featuresextending in a different direction (e.g., substantially perpendicular)to the direction in which the features of another periodic structure ofthe sub-target extend. In an embodiment, the direction(s) in whichfeatures of periodic structures of one sub-target extend may bedifferent from that of another sub-target.

In an embodiment, as shown in FIG. 9, each sub-target has a first pairof periodic structures 810 having features extending in a firstdirection (e.g., X-direction) and a second pair of periodic structures812 having features extending in a second different direction (e.g., asecond direction substantially perpendicular to the first direction suchas the Y-direction). As discussed above, one or more of the sub-targetsneed not have the second pair of periodic structures extend in adifferent direction or the second different direction may benon-perpendicular and non-parallel to the first direction for one ormore of the sub-targets. In this example, each sub-target 802-808 has asimilar overall layout as the target of FIG. 4. That is, each sub-targethas a first pair of periodic structures with features extending in theX-direction in opposite corners and a second pair of periodic structureswith features extending in the Y-direction in opposite corners to thefirst pair of periodic structures. However, the layout of thesub-targets may be different than as depicted in FIG. 9. For example,the locations of the periodic structures may be different. As anotherexample, the length and/or width of one pair of periodic structures maybe different than the length and/or width of another pair of periodicstructures. The relative angles in which one pair of periodic structuresextends to another pair of periodic structures may be different.Examples of different layouts for sub-targets are described with respectto FIGS. 12A-E.

The sub-targets 802-808 have a size such that they can fully or at leastpartly fit within the same contiguous area as the target of FIG. 4. Forexample, the extended operating range metrology target 800 may haveouter dimensions within or equal to 25 μm×25 μm, within or equal to 20μm×20 μm, within or equal to 16 μm×16 μm, within or equal to 12 μm×12μm, within or equal to 10 μm×10 μm, or within or equal to 8 μm×8 μm. Inan embodiment, at least part of each of sub-target is within acontiguous area of a certain size on the substrate. In an embodiment, atleast part of each periodic structure of the plurality of sub-targets iswithin the contiguous area of the certain size on the substrate. In anembodiment, each periodic structure of the plurality of sub-targets iswithin the contiguous area of the certain size on the substrate. In anembodiment, the certain size is less than or equal to 1000 μm², lessthan or equal to 900 μm², less than or equal to 800 μm², less than orequal to 700 μm², less than or equal to 600 μm², less than or equal to500 μm², less than or equal to 450 μm², less than or equal to 400 μm²,less than or equal to 350 μm², less than or equal to 300 μm², less thanor equal to 250 μm², less than or equal to 200 μm², less than or equalto 150 μm², or less than or equal to 100 μm². In an embodiment, each ofthe periodic structures of the sub-targets 802-808 is no smaller thanabout 3 μm×3 μm or no smaller than about 4 μm×4 μm. In an embodiment,each of the periodic structures of the sub-targets 802-808 is no smallerthan about 9 μm² or no smaller than about 16 μm².

In an embodiment, at least part of each of sub-target is within the areaof the measurement spot (e.g., within the width of the measurement spot)on the substrate. In an embodiment, at least part of each periodicstructure of the plurality of sub-targets is within the area of themeasurement spot (e.g., within the width of the measurement spot) on thesubstrate. In an embodiment, each periodic structure of the plurality ofsub-targets is within the area of the measurement spot (e.g., within thewidth of the measurement spot) on the substrate. In an embodiment, themeasurement spot has a width (e.g., diameter) of about 35 μm or less, ofabout 30 μm or less, of about 25 μm or less, or of about 20 μm or less,of about 15 μm or less, or of about 10 μm or less. So, in an embodiment,multiple sub-targets can be measured in one measurement sequence and inan embodiment, the diffraction data of multiple sub-targets can bedetected simultaneously.

Like with the target of FIG. 4, in an embodiment, a plurality of thesub-targets at least partly overlay another periodic structure (whichother periodic structure is not shown in FIG. 9 merely for clarity). Inan embodiment, each of the sub-targets 802-806 at least partly overlaysa respective periodic structure. In an embodiment, a first extendedoperating range metrology target 800 overlays a second extendedoperating range metrology target 800. In that case, each of theplurality of the sub-targets 802-806 of the first extended operatingrange metrology target 800 would overlay respective sub-targets 802-806of the second extended operating range metrology target 800. In anembodiment, the first extended operating range metrology target 800 maybe in one layer and the second extended operating range metrology target800 may be in one other layer. In an embodiment, the first extendedoperating range metrology target 800 may be in one layer and the secondextended operating range metrology target 800 may have each of aplurality of sub-targets in different layers.

Further, besides multiple sub-targets being created within a singlelayout, each of a plurality of the sub-targets is designed for (a) adifferent process condition, and/or (b) a different layer-pair formulti-layer overlay. In other words, in an embodiment, a firstsub-target 802 of the plurality of sub-targets has a different designthan a second sub-target 804 of the plurality of sub-targets. In anembodiment, each of the sub-targets 802-808 may have a different design.In an embodiment, two or more sub-targets 802, 808 of the plurality ofsub-targets may have a different design than two or more othersub-targets 804, 806 of the plurality of sub-targets.

Referring to FIG. 10, the use of an example of an extended operatingrange metrology target 900, 902 (of the design of FIG. 9) having aplurality of sub-targets designed for different process conditions isdepicted. For ease of reference, the sub-targets 802, 804, 806, 808 aredepicted in a row in FIG. 10. As will be appreciated from the layout ofFIG. 9, the sub-targets 806, 808 in FIG. 10 would in practice be locatedin “front” or “behind” the sub-targets 802, 804 in FIG. 10, i.e., in orout of the page respectively. Further, in this embodiment, the firstextended operating range metrology target 900 is at one layer and thesecond extended operating range metrology target 902 is at one otherlayer. That is, in FIG. 10, each of the sub-targets 802, 804, 806, 808of the first extended operating range metrology target 900 is at a toplayer and each of the sub-targets 802, 804, 806, 808 of the secondextended operating range metrology target 902 is in a single layerunderneath the first extended operating range metrology target 900, suchthat each of the sub-targets 802, 804, 806, 808 of the first extendedoperating range metrology target 900 at least partly overlays arespective sub-target 802, 804, 806, 808 of the second extendedoperating range metrology target 902.

In the example of FIG. 10, each of the sub-targets 802, 804, 806, 808 isdesigned for a different process stack. In this example, sub-target 802is designed for a process stack having a first layer 904 of 100 nm and asecond layer 906 of 100 nm, sub-target 804 is designed for a differentprocess stack having a first layer 904 of 100 nm and a second layer 906of 110 nm, sub-target 806 is designed for a different process stackhaving a first layer 904 of 110 nm and a second layer 906 of 110 nm, andsub-target 808 is designed for a process stack having a first layer 904of 120 nm and a second layer 906 of 110 nm. As will be appreciated, theconditions of the different process stacks may be different than thoseused in this example. For example, the process conditions can be otherthan layer thicknesses. Other process conditions may include refractiveindex, layer material, etch rate, bake temperature, exposure focus,exposure dose, etc. Further, while in this embodiment, the extendedoperating range metrology target 900 is differently designed than theassociated extended operating range metrology target 902 which itoverlays (e.g., in FIG. 10, periodic structure features in the extendedoperating range metrology target 902 are segmented, while periodicfeatures in the extended operating range metrology target 900 are not),the extended operating range metrology target 900 and the extendedoperating range metrology target 902 may be the same. Further, while 4different process stacks are capable of being successfully measured inFIG. 10, there may be a different number of process stacks that could becapable of being successfully measured.

In terms of difference in design, the difference is, in an embodiment, adifference in pitch of the periodic structures between at least one ofthe sub-targets 802, 804, 806, 808 and another of the sub-targets 802,804, 806, 808. In an embodiment, the pitch is selected from the range of100 nm to 1000 nm. In an embodiment, the difference in design is adifference in feature (e.g., line) or space width of the periodicstructures between at least one of the sub-targets 802, 804, 806, 808and another of the sub-targets 802, 804, 806, 808. In an embodiment, thedifference in design is a difference in segmentation of features of theperiodic structures (e.g., a broken line, rather than a solid line)between at least one of the sub-targets 802, 804, 806, 808 and anotherof the sub-targets 802, 804, 806, 808. In an embodiment, the differencein design is a difference in bias (e.g., amount and/or direction) of theperiodic structures between at least one of the sub-targets 802, 804,806, 808 and another of the sub-targets 802, 804, 806, 808. In anembodiment, the bias is selected in the range of 1 nm to 60 nm. Thearrows depict an embodiment of the direction of bias. To be sure a biasis not required. In an embodiment, the difference in design is adifference in feature or space width between overlying extendedoperating range metrology targets (e.g., a difference in “top and bottomCD”), e.g., a feature or space width of at least one of the sub-targets802, 804, 806, 808 of a first extended operating range metrology targetis different than the feature or space width of associated at least oneof the sub-targets 802, 804, 806, 808 of an overlying second extendedoperating range metrology target. In an embodiment, the difference indesign is a difference in layout of the sub-targets 802, 804, 806, 808and their associated periodic structures. See, e.g., FIGS. 12A-Edescribed hereafter. In an embodiment, the difference in design is adifference in optimum wavelength for the measuring beam between at leastone of the sub-targets 802, 804, 806, 808 and another of the sub-targets802, 804, 806, 808. Where the same wavelength measurement recipe is usedfor each of the sub-targets 802, 804, 806, 808, the sub-targets 802,804, 806, 808 may be optimized to accept a minimal performance loss oneach sub-target. Or, in an embodiment, multiple wavelengths may be usedfor the plurality of sub-targets or wavelengths may be separated out ofbroadband radiation applied to the sub-targets. As will be appreciated,a combination of design parameters may be used.

So, in an embodiment, the extended operating range metrology targets900, 902 may be provided, in a first example, to a process stack thathas the characteristics of sub-target 802, namely a process stack havinga first layer 904 of 100 nm and a second layer 906 of 100 nm.Accordingly, when the measurements of those extended operating rangemetrology targets 900, 902 are made, the measurement results fromsub-target 802 will be good for that process stack while the measurementresults from sub-targets 804, 806, and 808 will be less so. But,conveniently, the same extended operating range metrology targets 900,902 may be provided, in a second example, to a process stack that hasthe characteristics of sub-target 804, namely a process stack having afirst layer 904 of 100 nm and a second layer 906 of 110 nm. Accordingly,when the measurements of those extended operating range metrologytargets 900, 902 are made in this different process stack, themeasurement results from sub-target 804 in this case will be good forthat process stack while the measurement results from sub-targets 802,806, and 808 will be less so.

To determine whether the measurement results are good, one or moredifferent techniques may be used. For example, in the first examplementioned above, there may simply not be any or significantly weakermeasurement results from sub-targets 804, 806, and 808 because they areeffectively unmeasurable. In another example, a residual (e.g., anoverlay residual) can be measured for each of the sub-targets and alower or lowest residual for one of the sub-targets may signify that themeasurement results from the sub-target are good. In another example,the same parameter (e.g., overlay) may be measured by another process.As an example, an electrical test may be performed to determine a valuefor the parameter and the sub-target with the nearest value to thatmeasured by the electrical test may signify that the measurement resultsfrom the sub-target are good.

Referring to FIG. 11, the use of an example of an extended operatingrange metrology target 1000, 1002 (of the design of FIG. 9) having aplurality of sub-targets for multi-layer overlay is depicted. For easeof reference, the sub-targets 802, 804, 806, 808 are depicted in a rowin FIG. 11. As will be appreciated from the layout of FIG. 9, thesub-targets 806, 808 in FIG. 11 would in practice be located in “front”or “behind” the sub-targets 802, 804 in FIG. 11, i.e., in or out of thepage respectively. Further, in this embodiment, the first extendedoperating range metrology target 900 is at one layer and the secondextended operating range metrology target 902 has each of a plurality ofsub-targets in different layers. That is, in FIG. 11, each of thesub-targets 802, 804, 806, 808 of the first extended operating rangemetrology target 900 is at a top layer and each of the sub-targets 802,804, 806, 808 of the second extended operating range metrology target902 is in a different layer underneath the first extended operatingrange metrology target 900, such that each of the sub-targets 802, 804,806, 808 of the first extended operating range metrology target 900 atleast partly overlays a respective sub-target 802, 804, 806, 808 of thesecond extended operating range metrology target 902.

In the example of FIG. 11, each of the sub-targets 802, 804, 806, 808 isdesigned for a different layer. In this example, sub-target 802 isdesigned for measuring overlay for a first layer-pair of the top layerand layer 1010, sub-target 804 is designed for measuring overlay for asecond layer-pair of the top layer and layer 1008, sub-target 806 isdesigned for measuring overlay for a third layer-pair of the top layerand layer 1006, and sub-target 808 is designed for measuring overlay fora fourth layer-pair of the top layer and layer 1004. While eachsub-target in this example measures a different layer-pair, in anembodiment, two or more of the sub-targets may measure a firstlayer-pair and one or more other sub-targets may measure a secondlayer-pair. Further, while 4 different layer-pairs are capable of beingmeasured in FIG. 11, there may be a different number of layer-pairscapable of being measured.

In this embodiment, each of the sub-targets 802, 804, 806, 808 of thefirst extended operating range metrology target 900 has a same designand the sub-targets 802, 804, 806, 808 of the first extended operatingrange metrology target 900 is the same in terms of design as thesub-targets 802, 804, 806, 808 of the second extended operating rangemetrology target 902. However, as noted above, two or more of thesub-targets 802, 804, 806, 808 of the second extended operating rangemetrology target 902 are in different layers (and thus of differentdesign), while still underlying the first extended operating rangemetrology target 900. In an embodiment, one or more of the sub-targets802, 804, 806, 808 of the first extended operating range metrologytarget 900 may have a different design than another one or more of thesub-targets 802, 804, 806, 808 of the first extended operating rangemetrology target 900. In an embodiment, one or more of the sub-targets802, 804, 806, 808 of the first extended operating range metrologytarget 900 may have a different design than one or more of thesub-targets 802, 804, 806, 808 of the second extended operating rangemetrology target 902.

In an embodiment, because of the location of each of the sub-targets802, 804, 806, 808 in the extended operating range metrology target, theoverlay for each specific different layer-pair can be readily made.Moreover, since the extended operating range metrology target hassub-targets 802, 804, 806, 808 for each different layer-pair, themeasurement of a plurality of different layer-pairs may be taken in onemeasurement sequence, e.g., the diffraction information each of thedifferent layer-pairs may be captured at once. Instead of or in additionto using the measured overlay value of each different layer-pairseparately, the average, median or other statistical value of themeasurements using the sub-targets 802, 804, 806, 808 may be used forprocess control. This may be useful where there is a concern over thespecific reliability of one or more of the sub-targets 802, 804, 806,808 due their smallness. The statistical value can help eliminateanomalies.

FIGS. 12A-E depict further embodiments of an extended operating rangemetrology target. In an embodiment, these embodiments of extendedoperating range metrology target are designed for multi-layer overlaymeasurement. However, additionally or alternatively, these extendedoperating range metrology targets may be used, with appropriatemodifications, for process stack variation (i.e., different sub-targetsof the extended operating range metrology target are designed fordifferent process stack conditions). Of course, the design possibilitiesfor the extended operating range metrology target are not limited tothose depicted in FIGS. 9 and 12A-E. Different design variations of theextended operating range metrology target are possible to, e.g.,accommodate different or more process stack variations, differentamounts of layers, different layout constraints, etc. Further, each ofthe extended operating range metrology target designs in FIGS. 12A-Edepicts two sub-targets. As will be appreciated, the extended operatingrange metrology target may have more than two sub-targets.

In an embodiment, the extended operating range metrology target isdesigned to maximize the number of features exposed to radiation. In anembodiment, the extended operating range metrology target is designed tomaximize the same type of periodic structures (e.g., same dimensions,area, etc.). In an embodiment, the extended operating range metrologytarget is designed to maximize symmetry. In an embodiment, the extendedoperating range metrology target is designed to maximize the size ofperiodic structures of one sub-target against the size of periodicstructures of another sub-target while maintaining substantially thesame or similar diffraction efficiency for each of those sub-targets.

Referring to FIG. 12A, there is depicted an embodiment of an extendedoperating range metrology target 1200 having a first sub-target 1202 anda second sub-target 1204. Compared with the extended operating rangemetrology target of FIG. 9, the sub-targets are “interleaved” with eachother with in this case the periodic structures of the second sub-target1204 meeting at the center of the extended operating range metrologytarget 1200 and the periodic structures of the first sub-target 1202being arranged around the periphery. In this embodiment, the length L1and width W1 of each periodic structure of the first sub-target 1202 issubstantially the same as the length L2 (see FIG. 12B) and width W2 ofeach periodic structure of the second sub-target 1204. In an embodiment,the length L1, L2 is 8 μm and the width W1, W2 is 4 μm. In anembodiment, feature lengths are in the range of 3500-4000 nm, e.g., 3875nm. In an embodiment, the spacing between adjacent sides of the periodicstructures of the first and second sub-targets is in the range of150-400 nm, e.g., 250 nm. In an embodiment, the spacing is not uniformbetween all adjacent sides of the periodic structures of the first andsecond sub-targets. In an embodiment, there may be a bias differencebetween the first and second sub-targets 1202, 1204. The arrows depictan embodiment of the direction of bias. To be sure a bias is notrequired. In an embodiment, the bias is less than or equal to 60 nm. Inan embodiment, the extended operating range metrology target 1200 iscapable of measuring overlay in the range of 30 nm or less.

Referring to FIG. 12B, there is depicted an embodiment of an extendedoperating range metrology target 1220 having a first sub-target 1222 anda second sub-target 1224. Each of the sub-targets is a distinctcontiguous portion of the extended operating range metrology target1220. In this case, the first sub-target 1222 is in the “top” part andthe second sub-target 1224 is in the “bottom” part. In this embodiment,the length L1 and width W1 of each periodic structure of the firstsub-target 1222 is substantially the same as the length L2 and width W2of each periodic structure of the second sub-target 1224. In anembodiment, the length L1, L2 is 8 μm and the width W1, W2 is 4 μm. Inan embodiment, feature lengths are in the range of 3500-4000 nm, e.g.,3875 nm. In an embodiment, the spacing between adjacent sides of theperiodic structures of the first and second sub-targets is in the rangeof 150-400 nm, e.g., 250 nm. In an embodiment, the spacing is notuniform between all adjacent sides of the periodic structures of thefirst and second sub-targets. In an embodiment, there may be adifference in bias between the first and second sub-targets 1222, 1224.The arrows depict an embodiment of the direction of bias. To be sure abias is not required. In an embodiment, the bias is less than or equalto 60 nm. In an embodiment, the extended operating range metrologytarget 1220 is capable of measuring overlay in the range of 30 nm orless.

Referring to FIG. 12C, there is depicted an embodiment of an extendedoperating range metrology target 1240 having a first sub-target 1242 anda second sub-target 1244. The design of FIG. 12C is similar to thedesign of FIG. 12A in that the sub-targets are “interleaved” with eachother with in this case the periodic structures of the second sub-target1244 meeting at the center of the extended operating range metrologytarget 1240 and the periodic structures of the first sub-target 1242being arranged around the periphery. In this embodiment, the length L1of each periodic structure of the first sub-target 1242 is differentthan the length L2 of each periodic structure of the second sub-target1244 and the width W1 of each periodic structure of the first sub-target1242 is substantially the same as the width W2 of each periodicstructure of the second sub-target 1244. In an embodiment, the length L1is 6 μm and the width W1 is 4.9 μm. In an embodiment, the length L2 is10.4 μm and the width W2 is 4.9 μm. In an embodiment, feature lengthsare in the range of 3500-4000 nm, e.g., 3875 nm. In an embodiment, thespacing between adjacent sides of the periodic structures of the firstand second sub-targets is in the range of 150-400 nm, e.g., 250 nm. Inan embodiment, the spacing is not uniform between all adjacent sides ofthe periodic structures of the first and second sub-targets. In anembodiment, there may be a bias difference between the first and secondsub-targets 1242, 1244. The arrows depict an embodiment of the directionof bias. To be sure a bias is not required. In an embodiment, the biasis less than or equal to 60 nm. In an embodiment, the extended operatingrange metrology target 1240 is capable of measuring overlay in the rangeof 30 nm or less. This embodiment may be advantageous for multilayeroverlay where the second sub-target 1244 is used for a lower layer thanthe first sub-target 1242 because the nature of the layer material,thickness, etc. significantly attenuates or otherwise disturbs thediffracted radiation from the lower layer. Software for designing theextended operating range metrology target (described in more detailhereafter) can, based on the nature of the layer material, thickness,etc., choose the design parameters (e.g., feature and space width,pitch, layout, etc.) of the periodic structures of the first and secondsub-targets 1242, 1244 such that a diffraction efficiency of each of thefirst and second sub-targets 1242, 1244 is substantially the same orsimilar. This can help prevent clipping of a measurement sensor fromexcess diffracted radiation from the first sub-target 1242 or the secondsub-target 1244.

Referring to FIG. 12D, there is depicted an embodiment of an extendedoperating range metrology target 1260 having a first sub-target 1262 anda second sub-target 1264. The design of FIG. 12D is similar to thedesign of FIG. 12C with the difference that this design is moresymmetric. In this case, the second sub-target 1264 is in a cross-shapedform and the first sub-target 1262 is arranged around the periphery. Inthis embodiment, the length L1 of each periodic structure of the firstsub-target 1262 is different than the length L2 of each periodicstructure of the second sub-target 1264 and the width W1 of eachperiodic structure of the first sub-target 1262 is substantially thesame as the width W2 of each periodic structure of the second sub-target1264. In an embodiment, the length L1 is 5.4 μm and the width W1 is 5.4μm. In an embodiment, the length L2 is 7.5 μm and the width W2 is 5.4μm. In an embodiment, feature lengths are in the range of 3500-4000 nm,e.g., 3875 nm. In an embodiment, the spacing between adjacent sides ofthe periodic structures of the first and second sub-targets is in therange of 150-400 nm, e.g., 250 nm. In an embodiment, the spacing is notuniform between all adjacent sides of the periodic structures of thefirst and second sub-targets. In an embodiment, there may be a biasdifference between the first and second sub-targets 1262, 1264. Thearrows depict an embodiment of the direction of bias. To be sure a biasis not required. In an embodiment, the bias is less than or equal to 60nm. In an embodiment, the extended operating range metrology target 1260is capable of measuring overlay in the range of 30 nm or less. Thisembodiment may be advantageous for multilayer overlay where the secondsub-target 1264 is used for a lower layer than the first sub-target 1262because the nature of the layer material, thickness, etc. significantlyattenuates or otherwise disturbs the diffracted radiation from the lowerlayer. Software for designing the extended operating range metrologytarget (described in more detail hereafter) can, based on the nature ofthe layer material, thickness, etc., choose the design parameters (e.g.,feature and space width, pitch, layout, etc.) of the periodic structuresof the first and second sub-targets 1262, 1264 such that a diffractionefficiency of each of the first and second sub-targets 1262, 1264 issubstantially the same or similar. This can help prevent clipping of ameasurement sensor from excess diffracted radiation from the firstsub-target 1262 or the second sub-target 1264. This design is slightlymore balanced than the design of FIG. 12C.

Referring to FIG. 12E, there is depicted an embodiment of an extendedoperating range metrology target 1280 having a first sub-target 1282 anda second sub-target 1284. The design of FIG. 12E is similar to thedesigns of FIGS. 12C and 12D in that the periodic structures of thefirst and second sub-targets 1282 and 1284 differ. In the design of FIG.12E, the periodic structures of the first sub-target 1282 areconcentrated at the interior and the periodic structures of the secondsub-target 1284 are arranged around the periphery. In this embodiment,the length L1 and width W1 of each periodic structure of the firstsub-target 1282 is different than the length L2 and width W2 of eachperiodic structure of the second sub-target 1284. In an embodiment, thelength L1 is 6.25 μm and the width W1 is 6.25 μm. In an embodiment, thelength L2 is 12.5 μm and the width W2 is 7.5 μm. In an embodiment,feature lengths are in the range of 3500-4000 nm, e.g., 3875 nm. In anembodiment, the spacing between adjacent sides of the periodicstructures of the first and second sub-targets is in the range of150-400 nm, e.g., 250 nm. In an embodiment, the spacing is not uniformbetween all adjacent sides of the periodic structures of the first andsecond sub-targets. In an embodiment, there may be a bias differencebetween the first and second sub-targets 1282, 1284. The arrows depictan embodiment of the direction of bias. To be sure a bias is notrequired. In an embodiment, the bias is less than or equal to 60 nm. Inan embodiment, the extended operating range metrology target 1280 iscapable of measuring overlay in the range of 30 nm or less. Thisembodiment may be advantageous for multilayer overlay where the secondsub-target 1284 is used for a lower layer than the first sub-target 1282because the nature of the layer material, thickness, etc. significantlyattenuates or otherwise disturbs the diffracted radiation from the lowerlayer. Software for designing the extended operating range metrologytarget (described in more detail hereafter) can, based on the nature ofthe layer material, thickness, etc., choose the design parameters (e.g.,feature and space width, pitch, layout, etc.) of the periodic structuresof the first and second sub-targets 1282, 1284 such that a diffractionefficiency of each of the first and second sub-targets 1282, 1284 issubstantially the same or similar. This can help prevent clipping of ameasurement sensor from excess diffracted radiation from the firstsub-target 1282 or the second sub-target 1284. This design is slightlymore balanced than the design of FIG. 12C. Further, in this embodiment,the first sub-target 1282 may be smaller than the measurement spot(i.e., the first sub-target 1282 is overfilled), while the secondsub-target 1284 would be larger than the measurement spot (i.e., thesecond sub-target 1284 is underfilled). While underfilled, enough of thesecond sub-target 1284 may be captured to take a measurement.

Referring to FIGS. 22(A)-(C), the use of an example of an extendedoperating range metrology target 1500, 1502 having a plurality ofsub-targets for multi-layer overlay is depicted. In this embodiment,extended operating range metrology target 1500, 1502 comprisessub-targets 1504 and 1506. The sub-target 1504 comprises periodicstructures 1508, while sub-target 1506 comprises periodic structures1510.

In this example, FIG. 22(A) depicts the location of periodic structures1510 of sub-target 1504 in a lower layer, designated as layer 1. FIG.22(B) depicts the location of periodic structures 1512 of sub-target1506 in a higher layer, designated as layer 2, located above layer 1.FIG. 22(C) depicts the location of periodic structures of sub-targets1504 and 1506 in a higher layer, designated as layer 3, located abovelayers 1 and 2. The layers need not be immediately adjacent each other.For example, one or more other layers may be provided between layer 1and layer 2 or between layer 2 and layer 3, which other layers would nothave a periodic structure therein overlapping with any of the periodicstructures of FIGS. 22(A)-(C). In an embodiment, the extended operatingrange metrology target 1500, 1502 may have one or more furthersub-targets. In an embodiment, each of the one or more furthersub-targets may be located in respective one or more further layers (andthus allow for further layer-pairs to be measured).

Further, in practice, periodic structures in FIG. 22(C) would be atleast partly overlying periodic structures in FIG. 22(A) and periodicstructures in FIG. 22(C) would be at least partly overlying periodicstructures in FIG. 22(B). In particular, periodic structures 1510 inFIG. 22(C) would be at least partly overlying periodic structures 1510in FIG. 22(A). Further, periodic structures 1512 in FIG. 22(C) would beat least partly overlying periodic structures 1512 in FIG. 22(B). In anembodiment, the order of the periodic structures in the layers may bechanged. For example, FIG. 22(C) may be located at layer 2, while FIG.22(B) may be located at layer 3 (in which case FIG. 22(A) would be atlayer 1) or may be located at layer 1 (in which case FIG. 22(A) would beat layer 3). In this case, different layer-pair combinations can bemeasured, namely overlay between layers 1 and 2 and/or between layers 2and 3. Or, for example, FIG. 22(C) may be located at layer 1, while FIG.22(B) may still be located at layer 2 (and thus FIG. 22(A) would belocated at layer 3) or FIG. 22(B) may be located at layer 3 (in whichcase FIG. 22(A) would be located at layer 2).

In this embodiment, the features of the periodic structures 1510 ofsub-target 1504 extend in a first direction, which may be denominated asthe Y-direction. The periodic structures 1510 accordingly are able todetermine overlay in a second direction, which may be denominated as theX-direction, which is substantially orthogonal to the first direction.Further, the features of the periodic structures 1512 of sub-target 1506extend in the same first direction. Thus, the periodic structures 1512are likewise able to determine overlay in the X-direction.

In an embodiment, the features of the periodic structures 1510 ofsub-target 1504 extend in the second direction. In that case, theperiodic structures 1510 are able to determine overlay in theY-direction. Further, the features of the periodic structures 1512 ofsub-target 1506 would extend in the same second direction. Thus, theperiodic structures 1512 would likewise be able to determine overlay inthe Y-direction.

So, in the embodiment of FIG. 22, the extended operating range metrologytarget 1500, 1502 allows determination of overlay in the X-direction (orY-direction) between layer 1 (FIG. 22(A)) and layer 3 (FIG. 22(C)),while also allowing determination of overlay in the X-direction betweenlayer 2 (FIG. 22(B)) and layer 3 (FIG. 22(C)). Thus, in a singlemeasurement sequence, overlay in a same direction between differentlayer-pairs may be accomplished.

To facilitate checking of alignment of the periodic structures to helpensure that appropriate one or more periodic structures at least partlyoverlay associated one or more periodic structures, an optional marker1508 may be provided at each of a plurality of the layers. For example,a coarse alignment may be performed using the marker 1508 to, forexample, help ensure that periodic structures are generally overlyingother periodic structures (e.g., if one marker 1508 is considerablymisaligned from another, measurements may not be made using the target).Additionally or alternatively, the marker 1508 may be used to facilitatealignment of the measurement beam spot in the middle of the target.

Referring to FIGS. 23(A)-(C), the use of an example of an extendedoperating range metrology target 1600, 1602 having a plurality ofsub-targets for multi-layer overlay is depicted. In this embodiment,extended operating range metrology target 1600, 1602 comprisessub-targets 1604, 1606, 1608, 1610. The sub-target 1604 comprisesperiodic structures 1612, sub-target 1606 comprises periodic structures1614, sub-target 1608 comprises periodic structures 1616 and sub-target1610 comprises periodic structures 1618.

In this example, FIG. 23(A) depicts the location of periodic structures1614 of sub-target 1606 and periodic structures 1616 of sub-target 1608in a lower layer, designated as layer 1. FIG. 23(B) depicts the locationof periodic structures 1612 of sub-target 1604 and periodic structures1618 of sub-target 1610 in a higher layer, designated as layer 2,located above layer 1. FIG. 23(C) depicts the location of periodicstructures of sub-targets 1604, 1606, 1608, 1610 in a higher layer,designated as layer 3, located above layers 1 and 2. The layers need notbe immediately adjacent each other. For example, one or more otherlayers may be provided between layer 1 and layer 2 or between layer 2and layer 3, which other layers would not have a periodic structuretherein overlapping with any of the periodic structures of FIGS.23(A)-(C).

Further, in practice, periodic structures in FIG. 23(C) would be atleast partly overlying periodic structures in FIG. 23(A) and periodicstructures in FIG. 23(C) would be at least partly overlying periodicstructures in FIG. 23(B). In particular, periodic structures 1614 and1616 in FIG. 23(C) would be at least partly overlying respectiveperiodic structures 1614 and 1616 in FIG. 23(A). Further, periodicstructures 1612 and 1618 in FIG. 23(C) would be at least partlyoverlying respective periodic structures 1612 and 1618 in FIG. 23(B). Inan embodiment, the order of the periodic structures in the layers may bechanged. For example, FIG. 23(C) may be located at layer 2, while FIG.23(B) may be located at layer 3 (in which case FIG. 23(A) would be atlayer 1) or may be located at layer 1 (in which case FIG. 23(A) would beat layer 3). In this case, different layer-pair combinations can bemeasured, namely overlay between layers 1 and 2 and/or between layers 2and 3. Or, for example, FIG. 23(C) may be located at layer 1, while FIG.23(B) may still be located at layer 2 (and thus FIG. 23(A) would belocated at layer 3) or FIG. 23(B) may be located at layer 3 (in whichcase FIG. 23(A) would be located at layer 2).

In this embodiment, the features of the periodic structures 1612 ofsub-target 1604 extend in a first direction, which may be denominated asthe Y-direction. The periodic structures 1612 accordingly are able todetermine overlay in a second direction, which may be denominated as theX-direction, which is substantially orthogonal to the first direction.Further, the features of the periodic structures 1614 of sub-target1606, periodic structures 1616 of sub-target 1608 and periodicstructures 1618 of sub-target 1610 extend in the same first direction.Thus, the periodic structures 1614, 1616 and 1618 are likewiserespectively able to determine overlay in the X-direction.

In an embodiment, the features of the periodic structures 1612 ofsub-target 1604 extend in the second direction. In that case, theperiodic structures 1612 are able to determine overlay in theY-direction. Further, the features of the periodic structures 1614, 1616and 1618 would extend in the same second direction. Thus, the periodicstructures 1614, 1616 and 1618 would likewise be able to determineoverlay in the Y-direction.

So, in the embodiment of FIG. 23, the extended operating range metrologytarget 1600, 1602 allows determination of overlay in the X-direction (orY-direction) between layer 1 (FIG. 23(A)) and layer 3 (FIG. 23(C)),while also allowing determination of overlay in the X-direction betweenlayer 2 (FIG. 23(B)) and layer 3 (FIG. 23(C)). Moreover, in this case,the overlay in the X-direction (or Y-direction) would be measured atleast two times for each layer-pair due to one or more periodicstructures of at least two sub-targets being in each layer. For example,in an embodiment, overlay in the X-direction (or Y-direction) betweenlayers 1 and 3 is measured by each of at least sub-targets 1604 and1610. Similarly, for example, in an embodiment, overlay in theX-direction (or Y-direction) between layers 2 and 3 is measured by eachof at least sub-targets 1606 and 1608. Thus, in a single measurementsequence, overlay in a same direction between different layer-pairs maybe accomplished a plurality of times for each layer-pair. The overlayresults may be statistically combined (e.g., averaged) or combined byweighting (e.g., the overlay value measured for a layer-pair using onesub-target is weighed more than the overlay value for the layer-pairmeasured using another sub-target).

Referring to FIGS. 24(A)-(C), the use of an example of an extendedoperating range metrology target 1700, 1702 having a plurality ofsub-targets for multi-layer overlay is depicted. In this embodiment,extended operating range metrology target 1700, 1702 comprisessub-targets 1704 and 1706. The sub-target 1704 comprises periodicstructures 1708, while sub-target 1706 comprises periodic structures1710.

In this example, FIG. 24(A) depicts the location of periodic structures1708 of sub-target 1704 in a lower layer, designated as layer 1. FIG.24(B) depicts the location of periodic structures 1710 of sub-target1706 in a higher layer, designated as layer 2, located above layer 1.FIG. 24(C) depicts the location of periodic structures of sub-targets1704 and 1706 in a higher layer, designated as layer 3, located abovelayers 1 and 2. The layers need not be immediately adjacent each other.For example, one or more other layers may be provided between layer 1and layer 2 or between layer 2 and layer 3, which other layers would nothave a periodic structure therein overlapping with any of the periodicstructures of FIGS. 24(A)-(C).

Further, in practice, periodic structures in FIG. 24(C) would be atleast partly overlying periodic structures in FIG. 24(A) and periodicstructures in FIG. 24(C) would be at least partly overlying periodicstructures in FIG. 24(B). In particular, periodic structures 1708 inFIG. 24(C) would be at least partly overlying periodic structures 1708in FIG. 24(A). Further, periodic structures 1710 in FIG. 24(C) would beat least partly overlying periodic structures 1710 in FIG. 24(B). In anembodiment, the order of the periodic structures in the layers may bechanged. For example, FIG. 24(C) may be located at layer 2, while FIG.24(B) may be located at layer 3 (in which case FIG. 24(A) would be atlayer 1) or may be located at layer 1 (in which case FIG. 24(A) would beat layer 3). In this case, different layer-pair combinations can bemeasured, namely overlay between layers 1 and 2 and/or between layers 2and 3. Or, for example, FIG. 24(C) may be located at layer 1, while FIG.24(B) may still be located at layer 2 (and thus FIG. 24(A) would belocated at layer 3) or FIG. 24(B) may be located at layer 3 (in whichcase FIG. 24(A) would be located at layer 2).

In this embodiment, the features of the periodic structures 1708 ofsub-target 1704 extend in a first direction, which may be denominated asthe Y-direction. The periodic structures 1708 accordingly are able todetermine overlay in a second direction, which may be denominated as theX-direction, which is substantially orthogonal to the first direction.Further, the features of the periodic structures 1710 of sub-target 1706extend in the second direction. The periodic structures 1710 accordinglyare able to determine overlay in the Y-direction.

In an embodiment, the features of the periodic structures 1708 ofsub-target 1704 extend in the second direction. In that case, theperiodic structures 1708 are able to determine overlay in theY-direction. Further, in that case, the features of the periodicstructures 1710 of sub-target 1706 would extend in the same seconddirection. Thus, the periodic structures 1710 would likewise be able todetermine overlay in the Y-direction.

So, in the embodiment of FIG. 24, the extended operating range metrologytarget 1700, 1702 allows determination of overlay in the X-directionbetween layer 1 (FIG. 24(A)) and layer 3 (FIG. 24(C)), while alsoallowing determination of overlay in the Y-direction between layer 2(FIG. 24(B)) and layer 3 (FIG. 24(C)). Or, for example, by shifting FIG.24(B) to layer 1 and shifting FIG. 24(A) to layer 2, the extendedoperating range metrology target 1700, 1702 in that case would allow fordetermination of overlay in the Y-direction between layer 1 and layer 3,while also allowing determination of overlay in the X-direction betweenlayer 2 and layer 3. Thus, in a single measurement sequence, overlay indifferent directions between different layer-pairs may be accomplished.

Referring to FIGS. 25(A)-(C), the use of an example of an extendedoperating range metrology target 1800, 1802 having a plurality ofsub-targets for multi-layer overlay is depicted. In this embodiment,extended operating range metrology target 1800, 1802 comprisessub-targets 1804, 1806, 1810 and 1812. The sub-target 1804 comprisesperiodic structures 1812, sub-target 1806 comprises periodic structures1814, sub-target 1808 comprises periodic structures 1816 and sub-target1810 comprises periodic structure 1818.

In this example, FIG. 25(A) depicts the location of periodic structures1816 of sub-target 1808 and periodic structure 1818 of sub-target 1810in a lower layer, designated as layer 1. FIG. 25(B) depicts the locationof periodic structures 1812 of sub-target 1806 and periodic structures1814 of sub-target 1806 in a higher layer, designated as layer 2,located above layer 1. FIG. 25(C) depicts the location of periodicstructures of sub-targets 1804, 1806, 1808 and 1810 in a higher layer,designated as layer 3, located above layers 1 and 2. The layers need notbe immediately adjacent each other. For example, one or more otherlayers may be provided between layer 1 and layer 2 or between layer 2and layer 3, which other layers would not have a periodic structuretherein overlapping with any of the periodic structures of FIGS.25(A)-(C).

Further, in practice, periodic structures in FIG. 25(C) would be atleast partly overlying periodic structures in FIG. 25(A) and periodicstructures in FIG. 25(C) would be at least partly overlying periodicstructures in FIG. 25(B). In particular, periodic structures 1816 and1818 in FIG. 25(C) would be at least partly overlying associatedperiodic structures 1816 and 1818 in FIG. 25(A). Further, periodicstructures 1812 and 1814 in FIG. 25(C) would be at least partlyoverlying associated periodic structures 1812 and 1814 in FIG. 25(B). Inan embodiment, the order of the periodic structures in the layers may bechanged. For example, FIG. 25(C) may be located at layer 2, while FIG.25(B) may be located at layer 3 (in which case FIG. 25(A) would be atlayer 1) or may be located at layer 1 (in which case FIG. 25(A) would beat layer 3). In this case, different layer-pair combinations can bemeasured, namely overlay between layers 1 and 2 and/or between layers 2and 3. Or, for example, FIG. 25(C) may be located at layer 1, while FIG.25(B) may still be located at layer 2 (and thus FIG. 25(A) would belocated at layer 3) or FIG. 25(B) may be located at layer 3 (in whichcase FIG. 25(A) would be located at layer 2).

In this embodiment, the features of the periodic structures 1812 ofsub-target 1804 and periodic structures 1814 of sub-target 1806 extendin a first direction, which may be denominated as the Y-direction. Theperiodic structures 1812 and 1814 accordingly are able to respectivelydetermine overlay in a second direction, which may be denominated as theX-direction, which is substantially orthogonal to the first direction.Further, the features of the periodic structures 1816 of sub-target 1808and periodic structures 1818 of sub-target 1810 extend in the seconddirection. The periodic structures 1816 and 1818 accordingly are able torespectively determine overlay in the Y-direction.

In an embodiment, the features of the periodic structures 1812 ofsub-target 1804 and periodic structures 1814 of sub-target 1806 extendin the second direction. In that case, the periodic structures 1812 and1814 are able to determine overlay in the Y-direction. Further, in thatcase, the features of the periodic structures 1816 of sub-target 1808and periodic structures 1818 of sub-target 1810 would extend in thefirst direction. Thus, in that case, the periodic structures 1816 and1818 are able to determine overlay in the X-direction.

So, in the embodiment of FIG. 25, the extended operating range metrologytarget 1800, 1802 allows determination of overlay in the X-directionbetween layer 2 (FIG. 25(B)) and layer 3 (FIG. 25(C)), while alsoallowing determination of overlay in the Y-direction between layer 1(FIG. 25(A)) and layer 3 (FIG. 25(C)). Or, for example, by shifting FIG.25(B) to layer 1 and shifting FIG. 25(A) to layer 2, the extendedoperating range metrology target 1800, 1802 in that case would allow fordetermination of overlay in the X-direction between layer 1 and layer 3,while also allowing determination of overlay in the Y-direction betweenlayer 2 and layer 3. Moreover, in this case, the overlay in theX-direction and Y-direction) would be measured at least two times foreach layer-pair due to one or more periodic structures of at least twosub-targets being in each layer. For example, in an embodiment, overlayin the X-direction between layers 2 and 3 is measured by each of atleast sub-targets 1804 and 1806. Similarly, for example, in anembodiment, overlay in the Y-direction between layers 1 and 3 ismeasured by each of at least sub-targets 1808 and 1810. Thus, in asingle measurement sequence, overlay in different directions betweendifferent layer-pairs may be accomplished a plurality of times for eachlayer-pair. The overlay results may be statistically combined (e.g.,averaged) or combined by weighting (e.g., the overlay value measured fora layer-pair using one sub-target is weighed more than the overlay valuefor the layer-pair measured using another sub-target).

Referring to FIGS. 26(A)-(E), the use of an example of an extendedoperating range metrology target 1800, 1802 having a plurality ofsub-targets for multi-layer overlay is depicted. In this embodiment,extended operating range metrology target 1800, 1802 comprisessub-targets 1804, 1806, 1810 and 1812. The sub-target 1804 comprisesperiodic structures 1812, sub-target 1806 comprises periodic structures1814, sub-target 1808 comprises periodic structures 1816 and sub-target1810 comprises periodic structure 1818.

In this example, FIG. 26(A) depicts the location of periodic structures1814 of sub-target 1806 in a lower layer, designated as layer 1. FIG.26(B) depicts the location of periodic structures 1818 of sub-target1810 in a higher layer, designated as layer 2, located above layer 1.FIG. 26(C) depicts the location of periodic structures 1816 ofsub-target 1808 in a higher layer, designated as layer 3, located abovelayers 1 and 2. FIG. 26(D) depicts the location of periodic structures1812 of sub-target 1804 in a higher layer, designated as layer 4,located above layers 1-3. FIG. 26(E) depicts the location of periodicstructures of sub-targets 1804, 1806, 1808 and 1810 in a higher layer,designated as layer 5, located above layers 1-4. The layers need not beimmediately adjacent each other. For example, one or more other layersmay be provided between layer 1 and layer 2, between layer 2 and layer3, between layer 3 and layer 4 and/or between layer 4 and layer 5, whichother layers would not have a periodic structure therein overlappingwith any of the periodic structures of FIGS. 26(A)-(E).

Further, in practice, periodic structures in FIG. 26(E) would be atleast partly overlying periodic structures in FIG. 26(A), periodicstructures in FIG. 26(E) would be at least partly overlying periodicstructures in FIG. 26(B), periodic structures in FIG. 26(E) would be atleast partly overlying periodic structures in FIG. 26(C) and periodicstructures in FIG. 26(E) would be at least partly overlying periodicstructures in FIG. 26(D). In particular, periodic structures 1814 inFIG. 26(E) would be at least partly overlying periodic structures 1814in FIG. 26(A). Further, periodic structures 1818 in FIG. 26(E) would beat least partly overlying periodic structures 1818 in FIG. 26(B),periodic structures 1816 in FIG. 26(E) would be at least partlyoverlying periodic structures 1816 in FIG. 26(C) and periodic structures1812 in FIG. 26(E) would be at least partly overlying periodicstructures 1812 in FIG. 26(D). In an embodiment, the order of theperiodic structures in the layers may be changed. For example, FIG.26(E) may be located at layer 3, while FIG. 26(C) may be located atlayer 5 or another layer provided that structure that would otherwise beat that layer is moved to another layer. In this case, differentlayer-pair combinations can be measured, namely overlay between layers 1and 3, between layers 2 and 3, between layers 3 and 4 and/or betweenlayers 3 and 5. Or, for example, FIG. 26(E) may be located at layer 2,while FIG. 26(B) may be located at layer 5 or another layer providedthat structure that would otherwise be at that layer is moved to anotherlayer.

In this embodiment, the features of the periodic structures 1812 ofsub-target 1804 and periodic structures 1814 of sub-target 1806 extendin a first direction, which may be denominated as the Y-direction. Theperiodic structures 1812 and 1814 accordingly are able to respectivelydetermine overlay in a second direction, which may be denominated as theX-direction, which is substantially orthogonal to the first direction.Further, the features of the periodic structures 1816 of sub-target 1808and periodic structures 1818 of sub-target 1810 extend in the seconddirection. The periodic structures 1816 and 1818 accordingly are able torespectively determine overlay in the Y-direction.

In an embodiment, the features of the periodic structures 1812 ofsub-target 1804 and periodic structures 1814 of sub-target 1806 extendin the second direction. In that case, the periodic structures 1812 and1814 are able to determine overlay in the Y-direction. Further, in thatcase, the features of the periodic structures 1816 of sub-target 1808and periodic structures 1818 of sub-target 1810 would extend in thefirst direction. Thus, in that case, the periodic structures 1816 and1818 are able to determine overlay in the X-direction.

So, in the embodiment of FIG. 26, the extended operating range metrologytarget 1800, 1802 allows determination of overlay in the X-directionbetween layer 1 (FIG. 26(A)) and layer 5 (FIG. 26(E)) and between layer4 (FIG. 26(D)) and layer 5 (FIG. 26(E)), while also allowingdetermination of overlay in the Y-direction between layer 2 (FIG. 26(B))and layer 5 (FIG. 26(E)) and between layer 3 (FIG. 26(C)) and layer 5(FIG. 26(E)). Or, for example, by shifting FIG. 26(B) to layer 1 andshifting FIG. 26(A) to layer 2, the extended operating range metrologytarget 1800, 1802 in that case would allow for determination of overlayin the X-direction between layer 2 and layer 5, while also allowingdetermination of overlay in the Y-direction between layer 1 and layer 5.Or, for example, by shifting FIG. 26(C) to layer 4 and shifting FIG.26(D) to layer 3, the extended operating range metrology target 1800,1802 in that case would allow for determination of overlay in theX-direction between layer 3 and layer 5, while also allowingdetermination of overlay in the Y-direction between layer 4 and layer 5.Thus, in a single measurement sequence, overlay in different directionsbetween different layer-pairs may be accomplished.

Further, in the embodiments of FIGS. 24-26, sub-targets have beendescribed and shown as comprising periodic structures having features inone particular direction. This need not be the case. Rather, in FIGS.24-26, sub-targets may comprise one or more periodic structures havingfeatures in a first direction and comprise one or more periodicstructure having features in a second different direction. For example,in FIG. 24, sub-target 1704 may comprise a periodic structure 1708 and aperiodic structure 1710. Similarly, sub-target 1706 may comprise aperiodic structure 1708 and a periodic structure 1710. Similar groupingsmay be applied in FIGS. 25 and 26.

The extended operating range metrology target can thus open up a new wayof working with metrology targets in, e.g., the process developmentphase and multi-layer overlay measurement. In advanced nodes (with,e.g., difficult and varying processes and/or multiple layers formulti-patterning (e.g., double patterning)), device designers andmanufacturers are dynamically changing process stacks and/or usingmultiple layers and expect that metrology will work. The extendedoperating range metrology target can thus bring more process robustnessto metrology measurements and increase the chance of first-time-successof metrology on a relatively unknown process stack. For example, abenefit from measurement speed can be realized if at least part of eachof sub-target of the extended operating range metrology target is withinthe area of the measurement spot. If so, the extended operating rangemetrology target can, for example, increase the chance of first timesuccess with metrology on a process stack where process conditions maybe unknown. Further, the extended operating range metrology target canenable quick measurement of multiple layers and/or handle significantvariations in the process stack with reduced cost in the terms of target“real estate”, patterning device manufacture and/or throughput. And, theextended operating range metrology target may be used at developmentand/or manufacturing sites using existing metrology apparatus and nosensor hardware change may be required.

As described above, in an embodiment, there is provided a system andmethod to design the extended operating range metrology target. In anembodiment, the extended operating range metrology target should besuited to the different process stacks expected and/or the multilayeroverlay measurement desired. Further, the extended operating rangemetrology target should be able to cover for typical process variations(which are different than the significant differences from differentprocess stacks). Accordingly, in an embodiment, a design methodology isemployed to help ensure robustness of the extended operating rangemetrology target. That is, the extended operating range metrologytarget, including its sub-targets and its associated periodicstructures, can be designed by calculation and/or simulation usingprocess stack information to help ensure robustness of the extendedoperating range metrology target. In particular, for example, for anextended operating range metrology target for different process stacks,the robustness of each sub-target can be determined for the expectedtypical process variation associated with the particular differentprocess stack associated with the sub-target.

As alluded to, proposed metrology target designs may be subject totesting and/or simulation in order to confirm their suitability and/orviability, both from a printability and a detectability standpoint. In acommercial environment, good overlay mark detectability may beconsidered to be a combination of low total measurement uncertainty aswell as a short move-acquire-move time, as slow acquisition isdetrimental to total throughput for the production line. Modernmicro-diffraction-based-overlay targets (μDBO) may be on the order of10-20 μm on a side, which provides an inherently low detection signalcompared to 40×160 μm² targets such as those used in the context ofmonitor substrates.

Additionally, once metrology targets that meet the above criteria havebeen selected, there is a possibility that detectability will changewith respect to typical process variations, such as film thicknessvariation, various etch biases, and/or geometry asymmetries induced bythe etch and/or polish processes. Therefore, it may be useful to selecta target that has low detectability variation and low variation in themeasured parameter of interest (e.g., overlay, alignment, etc.) againstvarious process variations. Likewise, the fingerprint (printingcharacteristics, including, for example, lens aberration) of thespecific machine that is to be used to produce the microelectronicdevice to be imaged will, in general, affect the imaging and productionof the metrology targets. It may therefore be useful to ensure that themetrology targets are resistant to fingerprint effects, as some patternswill be more or less affected by a particular lithographic fingerprint.

Accordingly, in an embodiment, there is provided a method to design anextended operating range metrology target. In an embodiment, it isdesirable to simulate various extended operating range metrology targetdesigns in order to confirm the suitability and/or viability of one ormore of the proposed extended operating range metrology target designs.

In a system for simulating a manufacturing process involving lithographyand metrology targets, the major manufacturing system components and/orprocesses can be described by various functional modules, for example,as illustrated in FIG. 19. Referring to FIG. 19, the functional modulesmay include a design layout module 1300, which defines a metrologytarget (and/or microelectronic device) design pattern; a patterningdevice layout module 1302, which defines how the patterning devicepattern is laid out in polygons based on the target design; a patterningdevice model module 1304, which models the physical properties of thepixilated and continuous-tone patterning device to be utilized duringthe simulation process; an optical model module 1306, which defines theperformance of the optical components of the lithography system; aresist model module 1308, which defines the performance of the resistbeing utilized in the given process; a process model module 1310, whichdefines performance of the post-resist development processes (e.g.,etch); and metrology module 1312, which defines the performance of ametrology system used with the metrology target and thus the performanceof the metrology target when used with the metrology system. The resultsof one or more of the simulation modules, for example, predictedcontours and CDs, are provided in a result module 1314.

The properties of the illumination and projection optics are captured inthe optical model module 1306 that includes, but is not limited to,NA-sigma (σ) settings as well as any particular illumination sourceshape, where σ (or sigma) is outer radial extent of the illuminator. Theoptical properties of the photo-resist layer coated on a substrate—i.e.refractive index, film thickness, propagation and polarizationeffects—may also be captured as part of the optical model module 1306,whereas the resist model module 1308 describes the effects of chemicalprocesses which occur during resist exposure, post exposure bake (PEB)and development, in order to predict, for example, contours of resistfeatures formed on the substrate. The patterning device model module1304 captures how the target design features are laid out in the patternof the patterning device and may include a representation of detailedphysical properties of the patterning device, as described, for example,in U.S. Pat. No. 7,587,704. The objective of the simulation is toaccurately predict, for example, edge placements and CDs, which can thenbe compared against the target design. The target design is generallydefined as the pre-OPC patterning device layout, and will be provided ina standardized digital file format such as GDSII or OASIS.

In general, the connection between the optical and the resist model is asimulated aerial image intensity within the resist layer, which arisesfrom the projection of radiation onto the substrate, refraction at theresist interface and multiple reflections in the resist film stack. Theradiation intensity distribution (aerial image intensity) is turned intoa latent “resist image” by absorption of photons, which is furthermodified by diffusion processes and various loading effects. Efficientsimulation methods that are fast enough for full-chip applicationsapproximate the realistic 3-dimensional intensity distribution in theresist stack by a 2-dimensional aerial (and resist) image.

Thus, the model formulation describes most, if not all, of the knownphysics and chemistry of the overall process, and each of the modelparameters desirably corresponds to a distinct physical or chemicaleffect. The model formulation thus sets an upper bound on how well themodel can be used to simulate the overall manufacturing process.However, sometimes the model parameters may be inaccurate frommeasurement and reading errors, and there may be other imperfections inthe system. With precise calibration of the model parameters, extremelyaccurate simulations can be done.

In a manufacturing process, variations in various process parametershave significant impact on the design of a suitable target that canfaithfully reflect a device design. Such process parameters include, butare not limited to, side-wall angle (determined by the etching ordevelopment process), refractive index (of a device layer or a resistlayer), thickness (of a device layer or a resist layer), frequency ofincident radiation, etch depth, floor tilt, extinction coefficient forthe radiation source, coating asymmetry (for a resist layer or a devicelayer), variation in erosion during a chemical-mechanical polishingprocess, and the like.

A metrology target design can be characterized by various parameterssuch as, for example, target coefficient (TC), stack sensitivity (SS),overlay impact (OV), or the like. Stack sensitivity can be understood asa measurement of how much the intensity of the signal changes as overlaychanges because of diffraction between target (e.g., grating) layers.Target coefficient can be understood as a measurement of signal-to-noiseratio for a particular measurement time as a result of variations inphoton collection by the measurement system. In an embodiment, thetarget coefficient can also be thought of as the ratio of stacksensitivity to photon noise; that is, the signal (i.e., the stacksensitivity) may be divided by a measurement of the photon noise todetermine the target coefficient. Overlay impact measures the change inoverlay error as a function of target design.

Described herein is a computer-implemented method of defining ametrology target design for use in, e.g., a metrology system simulationor in a target manufacturing process simulation (e.g., includingexposing the metrology target using a lithographic process, developingthe metrology target, etching the target, etc.). In an embodiment, oneor more design parameters (e.g., geometric dimensions) for the targetcan be specified and further discrete values or a range of values can bespecified for the one or more design parameters. Further, a user and/orthe system may impose one or more constraints on one or more designparameters (e.g., a relationship between pitch and space width, a limiton pitch or space width, a relationship between feature (e.g., line)width (CD) and pitch (e.g., feature width is less than pitch), etc.)either in the same layer or between layers, based on, e.g., thelithographic process for which the target is desired. In an embodiment,the one or more constraints may be on the one or more design parametersfor which discrete values or a range has been specified, or on one ormore other design parameters.

FIG. 20 schematically depicts a computer-implemented method of definingan extended operating range metrology target design in accordance withan embodiment. The method includes, at block B1, providing a range or aplurality of values for each of a plurality of design parameters (e.g.,geometric dimensions) of a metrology target.

In an embodiment, a user of a metrology target design system may specifyone or more of the design parameters (e.g., geometric dimensions) forthe metrology target. For example, the user may specify that an extendedoperating range metrology target is desired. The user may furtherspecify the number of sub-targets of the extended operating rangemetrology target. Further, in an embodiment, the user may specify (e.g.,select) the discrete values or a range of values for each of one or moreof the design parameters of the extended operating range metrologytarget, one or more sub-targets thereof, and one or more periodicstructures of the sub-targets. For example, the user may select a rangeor a set of values for feature (e.g., line) width, space width, size ofthe extended operating range metrology target, pitch, etc. for theextended operating range metrology target. In an embodiment, where themetrology target comprises multiple periodic structures (gratings), orsegmented periodic structures (gratings), the user may select or providea range or set of values for other design parameters, e.g., sharedpitch.

In an embodiment, the design parameters may include any one or moregeometric dimensions selected from: pitch of a periodic structure of thetarget, periodic structure feature (e.g., line) width of the target,periodic structure space width of the target, one or more segmentationparameters of the features of the periodic structure (segmentationpitch/feature width/space width in X and/or Y direction depending onsegmentation type). Further, the parameters may be specified for asingle layer or a plurality of layers (e.g., two layers or two layersplus an intermediate shielding layer). For a plurality of layers, theymay share pitch. For certain metrology targets, e.g. focus or alignmenttargets, other parameters may be used. Other design parameters may bephysical limitations such as one or more selected from: a wavelength ofradiation used in the metrology system for the target, polarization ofradiation used in the metrology system, numerical aperture of themetrology system, target type, and/or a process parameter. In anembodiment, non-uniform and non-symmetric patterns, for examplemodulated overlay targets and focus targets, may be provided. Thus, thedesign parameters may be varied and not necessarily uniform in aparticular direction.

At block B2, there is provided one or more constraints for one or moredesign parameters of the metrology target. Optionally, the user maydefine one or more constraints. A constraint may be a linear algebraicexpression. In an embodiment, the constraint may be non-linear. Someconstraints may be related to other constraints. For example, featurewidth, pitch and space width are related such that if any two of thethree are known, the third may be fully determined.

In an embodiment, the user may specify a constraint on the area, adimension, or both, of the extended operating range metrology target.The user may specify a constraint on the number of sub-targets.

In an embodiment, a constraint may be a metrology parameter constraint.For example, in some metrology systems, the physics of the system mayplace a constraint. For example, a wavelength of radiation used in thesystem may constrain the pitch of the target design, e.g., a lowerlimit. In an embodiment, there is a (upper/lower) limit on pitch asfunction of wavelength, the type of target and/or the aperture of themetrology system. Physical limitations that can be used as constraintsinclude one or more selected from: a wavelength of radiation used in themetrology system, polarization of radiation used in the metrologysystem, numerical aperture of the metrology system, and/or target type.In an embodiment, the constraint may be a process parameter constraint(e.g., a constraint dependent on etch type, development type, resisttype, etc.).

Depending on the particular process being used, in an embodiment, one ormore constraints may be related to a constraint between a designparameter (e.g., geometric dimension) of one layer and a designparameter (e.g., geometric dimension) of another layer.

At block B3, by a processor, the method solves for and/or selects bysampling within the range or the plurality of values for the designparameters, a plurality of metrology target designs having one or moredesign parameters meeting the one or more constraints. For example, inan embodiment involving solving, one or more potential metrology targetsdesign may be solved for. That is, one or more potential metrologydesigns may be derived by solving for permitted values using, e.g., oneor more equality constraints to solve for specific values. For example,in an embodiment involving sampling, a convex polytope may be defined bythe various design parameters and constraints. The volume of the convexpolytope may be sampled according to one or more rules to provide samplemetrology target designs that meet all the constraints. One or moresampling rules may be applied to sample metrology target designs.

It is to be noted, however, that not all metrology target designs thusdiscovered are equally representative of process variations. As such, inan embodiment, the metrology target designs discovered using a methoddescribed herein may be further simulated, at block B4, to determine,for example, the viability and/or suitability of one or more of themetrology target designs. The simulated metrology target designs maythen be evaluated at block B5 to identify which one or more metrologytarget designs are best or more representative of process variation by,for example, ranking them based on a key performance index or arobustness criteria. At block B6, a particular metrology design may beselected and used, e.g., for measurement.

As noted above, it is desirable to make metrology targets (e.g., overlaytargets, alignment targets, focus targets, etc.) smaller. This is tolimit the “real-estate” consumption for metrology purposes on, e.g.,each production substrate. But with small size comes problems withdetection (e.g., image resolution).

In dark-field metrology, a single order of radiation may be transmittedto the detector, and creates a gray-level image of the target. Theindividual periodic structures are smaller than the illuminated area atread-out of the metrology targets and, therefore, the periodic structureedges are visible in the image. But, the periodic structure edges maypresent intensity levels that significantly deviate from the averageperiodic structure intensity. This is called an ‘edge effect’.

After a pattern-recognition step of the target in the dark-field image,a region-of-interest (ROI) is selected within the individual periodicstructures, which is used in the signal estimate. In this way, theaverage periodic structure intensity is extracted, while excluding theinfluence of edge effects. As such, the measured signal may be basedonly on a few detector pixels corresponding to the center of theperiodic structure in the image.

When a target is designed, the target design may be based on“infinitely” large periodic structures of which the feature-spacedimensions, pitch, sub-segmentation etc., is optimized. The periodicstructures may be positioned around predefined periodic structurecenters within the target. As a result, the target area is filled upmore or less efficiently depending on the pitch and feature-spacedimensions of the periodic structures.

In an embodiment, it is desirable to consider configuration (e.g.,optimization) of the entire target layout of an extended operating rangemetrology target with respect to optimum or improved detectability bythe metrology apparatus, including, e.g., optimized periodic structureto periodic structure distance, reduction of edge effects, andmaximization of the available grating area. Failure to configure foroptimum or improved detectability by the metrology apparatus may lead toone or more of the following issues:

-   -   1. Large edge effects at each periodic structure's periphery may        be observed in the dark field image. That may have one or more        of the following effects:        -   The size of available region of interest (ROI) may be            reduced (due to cropping of the image to exclude the            periodic structure edges), leading to a poor reproducibility            of the calculated signal.        -   The accuracy of the calculated periodic structure signal            (average intensity) may be reduced due to contamination of            the signal by optical crosstalk from emission due to edge            effects.        -   Instances of pattern recognition failure may be increased            due to a varying image with pronounced edge-effects over the            substrate and process variations in time.        -   The sensitivity of the calculated signal to ROI-positioning            errors may be increased; for example, the large edge            intensities may be accidentally included into the signal            estimation.        -   The use of the full-scale (of the full dynamic gray-level            range) of the detector may be reduced, leading to a reduced            reproducibility and sensitivity to systematic non-linear            sensor issues at low gray levels.    -   2. The total area comprising the periodic structures is not        maximized within the target region. Therefore, the maximum        photon count is not reached (e.g., not optimized for        reproducibility).

FIG. 13(a) gives an example of a target 700 layout comprising fourperiodic structures 720. Dashed shape 710 represents the availabletarget area. In FIG. 13(a), the target 700 layout is not optimized forthe available target area 710. The number of periodic structure featuresis calculated as a function of pitches and the available target area710. Subsequently, the predefined periodic structure features arecentered at the predetermined periodic structure midpoint. This resultsin non-optimized periodic structure to periodic structure distances(i.e. the space between periodic structures is not optimized within thetarget area). FIG. 13(b) illustrates a resultant dark field image 730following inspection of the target 700. Regions of medium/high intensitylevels 750 can be seen at the periodic structure positions. However, atthe periphery of the periodic structures, there are regions of evenhigher intensity levels 740 resultant from edge effects. This may makethe target difficult to analyze using a pattern recognition process,leading to failure-prone pattern recognition.

The measurement apparatus used to measure the target 700 effectivelyacts as a frequency band filter. When the measurement apparatus measuresa single periodic structure 720, it actually detects two structuretypes. The first structure is that comprising the repeating periodicstructure features, having a certain pitch. The second structure is theset of features seen as a single entity having a certain size (halfpitch); as these periodic structures are so small, they may be seen assingle structures as well as periodic structures. Both of these“structures” give their own sets of Fourier frequencies. If these twosets do not fit together they will create a step Fourier frequency set.The last frequency set has one or more frequencies that pass the bandfilter of the measurement apparatus. Unfortunately, the intensity ofthese frequencies is high thereby causing edge effects. In many casesthe edge effects result in intensities that are 2 to 4 times greaterthan the intensity of the maximum intensity grid.

To configure (e.g., optimize) target layout/designs for improvedmeasurement apparatus detection, embodiments described herein propose touse:

-   -   1. Configuration (e.g., optimization) of the target layout        taking into account the full available target area.    -   2. Computational lithography modeling using methods similar to        optical proximity correction (OPC) to configure (e.g., optimize)        the target layout for improved metrology process response (i.e.,        such configuration in addition to or alternatively to        configuration for improved or optimized ability to print the        target using a lithographic process). The resultant targets may        use one or more measurement tool-driven optical proximity        correction (MT-OPC) assist features to aid in improvement or        optimization of the metrology process response. In an        embodiment, the pitch and/or dimension of the MT-OPC assist        features is sub-resolution for the metrology apparatus.

For example, configuration of a target layout may begin by placing oneor more MT-OPC assist features at the periphery of the available targetarea, so as to ‘isolate’ the target from the environment and to reduceedge effects of periodic structures in a dark field image. The one ormore assist features are not observed in the dark field image capturedby the measurement apparatus, as their higher diffraction orders areusually not transmitted to the sensor (noting that the zeroth order isalso blocked).

Further, the available target area, inside of the one or more MT-OPCassist features, is filled with periodic structure features. For eachperiodic structure this may be done in the direction towards the center,beginning from the periphery. Periodic structure features may bepositioned in this way, while adapting their length, if needed, to fitcommensurately with the desired pitches and feature-space values of theneighboring periodic structure. One or more additional MT-OPC assistfeatures may be positioned between the periodic structures to reduceperiodic structure edge effects and separate the periodic structures inthe dark field image. Consequently, in an embodiment, each periodicstructure may have one or more MT-OPC assist features around its wholeperiphery. Such target layouts help to improve pattern recognition andto limit crosstalk. In an embodiment, the periodic structures of eachsub-target of an extended operating range metrology target may behandled separately such that, for example, the periodic structures ofone sub-target is processed as described above before the periodicstructure of another sub-target.

Thus, the configuration of a full target design may comprise:

-   -   1. Configuration (e.g., optimization) of the periodic structures        with respect to design restrictions. Such design restrictions        depend on the application given a specific product design, for        example: feature widths, sub-segmentation, “line on line” or        “line on trench”, etc.    -   2. Configuration of the whole target layout for improved or        optimum metrology process detection, in some cases using one or        more MT-OPC assist features. Sub-segmentation and/or other        design restrictions may be applied to the MT-OPC assist        features, where appropriate.    -   3. Performing one or more lithography OPC cycles to the entire        target layout to help ensure that the desired target layout        devised in steps 1 and 2 can be properly printed.

Configuration of the target may include configuration of any parameteror aspect of the target. This can include, for example, periodicstructure pitch, MT-OPC assist feature pitch, length and width of anyfeature, periodic structure duty cycle, etc. The configuration processtakes into account the entire available target region. In addition to oralternatively to using one or more MT-OPC assist features, the lengthand dimension (e.g., CD) of one or more periodic structure featuresadjacent to the gap between adjacent periodic structures may bemodified. For example, the length of periodic structure featuresextending toward the gap may be shortened or lengthened. As anotherexample, a periodic structure feature extending along the gap may haveits dimension narrowed or widened relative to other features of thatperiodic structure.

A potential target layout may be evaluated in a suitable measurementapparatus simulation tool. It may require several iterations to arriveat a desired (e.g., optimum) target layout specific for the measurementapparatus configuration. For example, in each iteration, theconfiguration of the target layout may be altered to help achieveimproved or optimum metrology process detection by, for example,changing the size, placement, number of features, pitch, etc. of the oneof more MT-OPC assist features. As will be appreciated, such change inthe configuration may be done automatically by the software and/orguided by a user. In an embodiment, the simulation takes account of thedifferent layers of an extended operating range metrology target (e.g.,in terms of different refractive indices, thicknesses, etc.). In anembodiment, the simulation takes accounts of a difference in pitch,feature dimension (CD), etc. between sub-targets.

Thus, desirably, this configuration may be carried out in an automatedfashion. An ‘automated’ method includes (not exclusively) (i) one ormore accurate optical models that can predict accurately the measurementapparatus response, within an acceptable timeframe and (ii) well-definedcriteria for configuration. For example, criteria may include one ormore selected from the following:

-   -   Periodic structure edge intensities having the same order of        magnitude as the periodic structure center intensities.    -   Minimum variation of edge effects in the presence of overlay,        defocus and/or aberrations of the measurement apparatus. In an        embodiment, robustness for the measurement recipe (e.g.,        wavelength, focus, etc.)    -   Sufficient spacing between the periodic structures for improved        or optimum target pattern recognition, for the relevant        wavelength range (e.g., spacing λ/2, with λ representing the        measurement radiation wavelength). For example, at least 1 line        of sensor pixels between the adjacent periodic structure regions        that exceed an intensity threshold.    -   Maximum periodic structure area.        Ideally these criteria are balanced in devising the final target        arrangement.

FIG. 14 shows examples of an extended operating range metrology targetsimilar to the design of FIG. 12A. Of course, in an embodiment, adifferent design of an extended operating range metrology may be used,such as the design of FIG. 9 or of any of FIGS. 12B-E.

FIG. 14(a) shows an example non-optimized target layout 1200 of anextended operating range metrology target comprising two sub-targets1202 and 1204. The non-optimized target layout 1200 also comprises fourperiodic structures 1400, each comprising, in this case, a portion ofsub-targets 1202 and 1204. Each periodic structure 1400 comprises aplurality of periodic structure features (e.g., grating lines). Thenumber of periodic structure features is calculated as a function ofpitch and the total predetermined grating area. Further, the predefinedperiodic structure features are centered at the predetermined periodicstructures midpoint. This results in non-matching and non-optimizedperiodic structure-to-periodic structure distances for metrologyapparatus observation. FIG. 14(c) illustrates an example simulation of adark field image that may result from the target layout of FIG. 14(a),and which shows clearly visible edge effects. These edge effects can beseen as regions of very high intensity measurements 1430 at theperiphery of the periodic structure regions 1440. In FIGS. 14(c) to14(f), regions with darker shading indicate higher intensities. FIG.14(e) illustrates a further example simulation of a dark field imagethat may result from the target layout of FIG. 14(a), using a differentwavelength than for the example of FIG. 14(c). It can be seen that theimages of the periodic structures in FIG. 14(e) are not clearlydelineated and so would not be easily recognized.

FIG. 14(b) shows an improved version of the target layout 1200 of FIG.14(a), comprising identical periodic structures 1400 to that of the FIG.14(a) arrangement, and further comprising one or more MT-OPC assistfeatures 1410, 1420. A first set of one or more MT-OPC assist features1410 is located at the periphery of the target (e.g., so as to surroundit), and a second set of one or more MT-OPC assist features 1420 islocated between a plurality of the periodic structures 1400. In anembodiment, each periodic structure 1400 is surrounded by a combinationof one or more MT-OPC assist features 1410, 1420. FIG. 14(d) illustratesan example simulation of a dark field image that may result from thetarget layout of FIG. 14(b), which shows reduced edge effects. FIG.14(f) illustrates a further example simulation of a dark field imagethat may result from the target layout of FIG. 14(b), using a differentwavelength than for the example of FIG. 14(d). It can be seen that theimages of the periodic structures in FIG. 14(e) are fairly clearlydelineated and so should be easily recognized.

Thus, a comparison of FIGS. 14(c) and 14(d) shows a far more uniformintensity distribution in the region of each periodic structure, withfewer edge effects, in FIG. 14(d). A comparison of FIGS. 14(e) and 14(f)shows enhanced dark field image resolution of FIG. 14(f) compared toFIG. 14(e), with improved separation of the periodic structures (i.e.,lower intensity between periodic structures in FIG. 14(f) when comparedto FIG. 14(e)), thus improving dark field pattern recognition.

In this example, the one or more MT-OPC assist features have a smallpitch, for example, of the order of 160 nm, resulting in evanescentwaves. The one or more MT-OPC assist features provide edge effectreduction and separation of the periodic structure from the environment.

FIG. 15 illustrates a magnified, partial view of a cross section of atarget 1200 comprising a periodic structure 1400 and one or more MT-OPCassist features 1420. In an embodiment, the one or more MT-OPC assistfeatures 1420 are positioned in the periodic structurespace-feature-space rhythm, avoiding abrupt steps (e.g., sharp,rectangular window). In this way, the one or more assist features 1420are positioned close to the periodic structure 1400 lines, whilebreaking the excitations within the periodic structure resultant fromits finite dimensions (e.g., softening the edges). FIG. 15 shows arepresentation of such matched positioning of the basic frequencies inthe periodic structures features and in the one or more MT-OPC assistfeatures, next to the neighboring 90°-rotated periodic structure. Inthis example, the pitch of the MT-OPC assist features is such thatdiffraction orders associated with the MT-OPC assist features are nottransmitted to the detector. While FIG. 15 shows the periodic structureof the one or more MT-OPC assist features 1420 having two features, itwill be appreciated it may have just one feature or more than twofeatures.

Ensuring that the periodic structure 1400 and one or more MT-OPC assistfeatures 1420 are in phase with each other helps avoid the “stepfrequency set” that causes high-intensity edge effects. The periodicstructure 1400 and the one or more MT-OPC assist features 1420 being inphase means that the one or more MT-OPC assist features 1420 extend thecontinuous surface of the periodic structure 1400. While there still areedge effects, those of high intensity are outside of the transmissionband of the measurement apparatus and are not detected by it. In thisway intensity peaks actually measured by the measurement apparatus arereduced. Thus, in an embodiment, the one or more MT-OPC assist featuresare strongly coupled to the periodic structures with a spectrum outsidethe transmission band to the measurement detector.

In an embodiment, the MT-OPC assist features should be in phase with itsassociated measurement periodic structure(s) but the designs for imagingthe periodic structure and measuring the periodic structure may makethis not possible. As an example, the design of the sub-targets of anextended operating range metrology target may get into problems withfitting the sub-targets in its constrained area as well as fitting theone or more assist features at the periphery of the sub-targets orbetween adjacent sub-targets. This issue with MT-OPC assist features maybe solved by providing a disruption in the middle of the MT-OPC assistfeature. For example, where the MT-OPC assist feature comprises aperiodic structure of three or more features, one or more of the middlefeatures may be enlarged. Similarly, where the MT-OPC assist featurecomprises a periodic structure of two or more features, one or more ofthe middle spaces between features may be enlarged. Consequently, thearea consumed by the MT-OPC assist feature may be enlarged. Theenlargement of the feature and/or space may be other than the middle.The enlargement of the feature and/or space and its location aredesigned so as to facilitate improved (e.g., as best as possible)matching of the phase.

In an embodiment, for one or more assist features located betweenadjacent periodic structures, the gap between the periodic structures isthe same as or about the same as the cross-wise dimensions (CD) of thefeatures of one or both of the adjacent periodic structures. In anembodiment, for one or more assist features located between adjacentperiodic structures, the cross-wise dimension of the spaces between theone or more assist features and the adjacent periodic structures isequal or about equal and in an embodiment, is equal or about equal tothe cross-wise dimension between a plurality of the assist features.

In an embodiment, optical waves diffracted from these one or more assistfeatures 1420 nominally do not carry any energy (evanescent ordestructively interfering), or are outside the part of the spectrum thatis transmitted to the detector (blocked propagating waves). In thisexample, incident radiation I, diffracted zeroth order radiation 0 andfirst order radiation −1 is shown. The −1 order radiation diffracted bythe one or more assist features 1420 is blocked, and only −1 orderradiation diffracted by periodic structure 1400 is transmitted to thesensor. However, due to the finiteness of the one or more assistfeatures 1420, the ‘tails’ of the assist feature reflections may leakinto the spectrum transmitted to the sensor and will interact with thespectrum from the periodic structure features.

For well separated periodic structures in the dark field image, the oneor more MT-OPC assist features 1420, in an embodiment, fill a spacebetween the periodic structures that has a width that is at least halfthe wavelength of the measurement apparatus. The same holds for theseparation and crosstalk reduction from the environment on the target.

FIG. 16(a) shows a target arrangement of an extended operating rangemetrology target 1600, the target comprising two sub-targets 1202 and1204. The target 1600 also comprises four periodic structures 1650, eachcomprising, in this case, a portion of sub-targets 1202 and 1204. Thetarget 1600 occupies, in practice, an area 1610. The target layoutincludes a ‘clearance’ region 1620 at the target boundaries, to improvedark field pattern recognition and reduce crosstalk from theenvironment. In FIG. 16(b), the target layout of FIG. 16(a) is replacedby a target layout 1630 optimized for the entire target area 1610. Thetarget layout includes one or more MT-OPC assist features 1635 atlocations around its periphery, and further one or more MT-OPC assistfeatures 1640 between a plurality of the periodic structures 1650. TheMT-OPC assist features 1635, 1640 help ensure dark field patternrecognition performance and optical crosstalk reduction from theenvironment, such that the ‘clearance’ region 1620 will not be needed.Therefore, the size, number of features and pitch of each periodicstructure 1650 can be configured to the available target area 1610.Corresponding dark field image simulation results (not illustrated)would show a strong reduction in edge effects, while pattern recognitionwould be improved by the periodic structure to periodic structureseparation.

FIG. 17 is a flowchart illustrating a method of devising an extendedoperating range metrology target arrangement according to an embodiment.The method comprises:

-   -   Step T1—Provide one or more MT-OPC assist features, with        ‘sub-resolution’ pitch and/or dimension, for example, near the        boundary and/or inside a design target region. This defines an        ‘available/empty’ design target region. The characteristics of        the one or more assist features (e.g. feature width, shape . . .        ) may be chosen, for example, to efficiently isolate the target        from the environment in the dark field image.    -   Step T2—Based on the one or more MT-OPC assist features placed        at the target boundary, place periodic structure features of a        first periodic structure (comprising features of one or more of        the sub-targets of the extended operating range metrology        target) sequentially in a direction towards the inside of the        target region, beginning at the boundary. For example, place        features until part of the last placed feature is located over a        halfway point of the available target area in the periodic        structure direction.    -   Step T3—Add one or more MT-OPC assist features (if needed),        having a form based on the size and pitch of adjacent periodic        structure features, and further having a ‘sub-resolution’ pitch        and/or dimension.    -   Step T4—Based on the latter one or more MT-OPC assist features,        adapt the feature length, if applicable, of the next periodic        structure to the remaining available target region.    -   Step T5—Repeat steps T2-T4 for the remaining periodic        structures.    -   Step T6—Optionally, fill-up the central part of target region        with one or more MT-OPC assist features.

An example application of this method is illustrated in FIG. 18. FIG.18(a) corresponds to step T1. One or more MT-OPC assist features 1810are drawn close to the border of an available target region, with apitch chosen to isolate the target from the environment and to reduceperiodic structure edge effects. FIGS. 18(b) and (c) correspond to stepT2, with periodic structure feature 1820 placed so as to fill upapproximately one quarter of the target region allocated to thisperiodic structure. FIG. 18(d) corresponds to step T3, with one or morefurther MT-OPC assist features 1830 added, matched to the adjacentperiodic structure features. FIG. 18(d) also illustrates the beginningof step T4, with the length of feature 1840 having been adapted to theremaining available area. FIG. 18(e) corresponds to an intermediatepoint during step T5, with two periodic structures placed and a thirdbegun. FIG. 18(f) illustrates the completed target arrangement, with oneor more additional MT-OPC assist features 1850 placed within a centralregion of the target layout as described in step T6. This method mayrequire several iterations, with each target arrangement obtained atstep T6 being evaluated using a metrology simulation tool. Evaluationmay comprise determining whether a particular arrangement meets one ormore predefined criteria and/or comparing multiple differentarrangements devised in accordance with this method so as to determinethe best one (based on one or more predefined criteria).

Instead of filling the central region of the target with one or moreadditional MT-OPC assist features 1850, this region could be filled witha special target (cross) for performing patterning device writingquality measurements.

Overlay metrology involves two stacked periodic structures (i.e., a twolayer target). For such targets, the lower target layout may be devisedusing the method of FIG. 18. The upper target layout usually contains anoverlay bias, ranging from five to several tens of nanometers. In suchan arrangement, the upper target layout may simply match the lowertarget layout, with the exception of the bias(es). In an example, thebias may be applied to only the periodic structure features in the uppertarget layout, with no bias applied to the one or more MT-OPC assistfeatures in the upper target layout. In an example, MT-OPC assistfeatures may be omitted from the upper target layout, which may helpavoid generation of an asymmetric signal perturbing the measurement, andis especially applicable if the back-reflected diffraction of the upperperiodic structure is weak and the main back-reflected diffractionoriginates from the lower periodic structure.

For “line-on-trench” instead of “line-on-line” target configurations,the upper target layout may be inverted, to obtain the “line-on-trench”configuration. For duty-cycles which differ from 50%, it is possible todesign the upper target layout as the “line-on-line” version with areverse duty cycle (100%—duty-cycle), which is then inverted to obtainthe “line-on-trench” configuration. The design of MT-OPC assist featuresin case of duty-cycle differences between upper and lower target layoutsmay lead to a more complex layout configuration procedure, however,those skilled in the art will be able to implement and customize thepresent method for such arrangements.

To help ensure printability and compliance with semiconductormanufacturer design rules, the dimensions of the one or more MT-OPCassist features may allow sub-segmentation of the one or more MT-OPCassist features.

The dimensions and/or shape of the one or more MT-OPC assist featuresmay be customized to the needs of the application. For instance, in theexample of FIG. 15, the MT-OPC assist features 1420 are represented by‘continuous rectangular’ shapes. However, continuous rectangular shapesmay lead to electric charging effects on a reticle or a printed circuitat sharp edges. To overcome this issue, shapes edges may be ‘deleted’from the layout.

In the above mentioned examples, the one or more MT-OPC assist featuresare ‘sub-resolution’ (i.e., have a smaller resolution than that ofproduct features). However, the one or more MT-OPC assist features mayhave dimensions below, within or above the resolution of the sensor,depending on the application. For example, the one or more outer MT-OPCassist features can be adapted to structures located in the area aroundthe target (e.g., device features). Where the pitch of the featuresaround the target is below or outside the range of detection of themetrology apparatus or the features around the target are about the samesize as the MT-OPC assist feature, then there may be no change in theMT-OPC assist feature required. Where the pitch of the features in thearea around the target is within the range of detection of the metrologyapparatus or the features around the target are not about the same sizeas the nominal MT-OPC assist feature, then the MT-OPC assist feature maybe changed in size (e.g., larger) to suppress the effect of one or morefeatures in the area around the target.

The present method for configuring a target layout/design may beapplied, for example, during design/configuration processes of metrologytargets for all metrology applications (including alignment). Forexample, the present method may be applied to alignment targets used inoverlay correction systems and/or in advanced alignment systems.

As shown in the above examples, one or more MT-OPC assist features maybe placed at the target boundary and/or may be placed around a periodicstructure in order reduce edge effects. In addition to this, one or moreMT-OPC assist features may be placed between the periodic structurefeatures (e.g. for a large pitch periodic structure such as an alignmentperiodic structure) in order to sharpen or soften feature-spacetransitions. This may help enhancement of the diffraction efficiencyinto desirable orders by improving or optimizing the intrinsicdiffraction efficiency for the detected orders, or improving oroptimizing the ordering of energy into the relevant diffraction orders.This may aid detectability for low ‘substrate quality’ stacks.Furthermore, the gain set point in alignment sensor electronics may beimproved, particularly for low substrate quality stacks, during theread-out and scanning over the target.

Further, the one or more MT-OPC assist features need not be located inthe same layers as a periodic structure. For example, the one or moreMT-OPC assist features may be located in a different layer desirablyabove, but also possibly below, the applicable periodic structure.Having the MT-OPC assist features in a different layer may facilitatemanufacturability (e.g., the MT-OPC assist features may not be printableusing the projection settings use to print the periodic structure, whichmay be the projection settings to print a device pattern).

Further, while the embodiment of MT-OPC assist features have beendescribed as specific uniform periodic structures adjacent or betweenthe target periodic structures, the MT-OPC assist features may takeother forms. For example, the assist features may take a form asdescribed in United States Patent Application Publication No.2013-0271740, which is incorporated herein in its entirety by reference.

The present method may be combined with current methods for improvingparameter estimation in, for example, dark field metrology.

The methods disclosed above result in larger ROIs and consequently,larger photon counts during intensity measurements. This improves thereproducibility for a constant target region. Improved reproducibilitymay also result from the reduction of edge effects, reducing inaccuracyin ROI positioning. In addition, reduction of edge effects improvespattern recognition as a consequence of a better defined dark fieldtarget image. Furthermore, the full gray scale dynamic range of themeasurement apparatus sensor may be used as edge effects will notsaturate the dark field image. Consequently, reproducibility is furtherimproved and non-linear sensor effects which result from photon noise atlow intensities are avoided. Photon noise is the square root of thenumber of measured photons. The number of measured photons is theproduct of the number of used pixels, the gray level and thesensitivity. To obtain a more stable measurement either the number ofpixels or number of gray levels needs to be increased; sensorsensitivity is fixed. By using one or more MT-OPC assist features, moregray levels can be obtained.

Adding one or more MT-OPC assist features to individual periodicstructures improves the isolation from the in-die environment whendistributing each periodic structure separately among device structures.The flexibility for in-die placement of the targets/periodic structuresis therefore improved due to the isolation of the periodic structuresfrom the surroundings.

Further, by the use of one or more MT-OPC assist features, the targetregion may be reduced (i.e. smaller target dimensions) while keeping asame reproducibility. Reduced target dimensions enable denserintra-field measurements. This improves, e.g., higher order overlaycorrections over the die on on-product substrate and lithographicapparatus performance characterization.

An embodiment of one or more these MT-OPC assist feature techniques maybe implemented at, for example, module 1312 of FIG. 19 and/orimplemented at blocks B2-B5 of FIG. 20.

FIG. 21 shows a flowchart illustrating a process in which the extendedoperating range metrology target is used to monitor performance, and asa basis for controlling metrology, design and/or production processes.In step D1, substrates are processed to produce product features and oneor more extended operating range metrology targets as described herein.At step D2, lithographic process parameter (e.g., overlay) values aremeasured and calculated using, e.g., the method of FIG. 6. At step D3,the measured lithographic process parameter (e.g., overlay) value isused (together with other information as may be available), to update ametrology recipe. The updated metrology recipe is used forre-measurement of the lithographic process parameter, and/or formeasurement of the lithographic process parameter on a subsequentlyprocessed substrate. In this way, the calculated lithographic processparameter is improved in accuracy. The updating process can be automatedif desired. In step D4, the lithographic process parameter value is usedto update a recipe that controls the lithographic patterning step and/orother process step in the device manufacturing process for re-workand/or for processing of further substrates. Again this updating can beautomated if desired.

While the embodiments of extended operating range metrology targetdescribed herein have mostly been described in the terms of overlaymeasurement, the embodiments of the extended operating range metrologytarget described herein may be used to measure one or more additional oralternative lithographic process parameters. For example, the extendedoperating range metrology target may be used to measure exposure dosevariation, measure exposure focus/defocus, etc. Thus, in an embodiment,the same extended operating range metrology target may be used tomeasure a plurality of different parameters. For example, the extendedoperating range metrology target may be arranged to measure overlay andmeasure one or more other parameters such as critical dimension, focus,dose, etc. As an example, one or more of the sub-targets may be designedto measure overlay (e.g., have their associated periodic structures indifferent layers) and one or more other sub-targets may be designed tomeasure critical dimension, and/or focus, and/or dose, etc. In anembodiment, a particular sub-target may be designed to measure two ormore parameters, e.g., overlay and one or more other parameters such ascritical dimension, focus, dose, etc.

In an embodiment, the periodic structures are desirably longer than wideas, for example, shown in FIG. 14(A). FIG. 14(A) depicts each of theperiodic structures of sub-target 1202 and 1204 as being longer than itswidth. Such an arrangement helps reduce crosstalk between the X and Ydirections. For smaller periodic structures as desired for, e.g., anextended operating range metrology target, the crosstalk tends to bestronger because the ratio between grating sides and total surface areais larger. The area that causes crosstalk is 0.5 times the wavelengthtimes the grating side times 2. Accordingly, longer than wide periodicstructures tend to reduce cross-talk and thus may be more advantageous.

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetperiodic structure’ as used herein do not require that the structure hasbeen provided specifically for the measurement being performed. Further,pitch P of the metrology target is close to the resolution limit of theoptical system of the scatterometer, but may be much larger than thedimension of typical product features made by lithographic process inthe target portions C. In practice the features and/or spaces of theoverlay periodic structures may be made to include smaller structuressimilar in dimension to the product features.

In certain embodiment, the periodic structures of the sub-targets of anextended operating range metrology target may be rotationally symmetric.That is there may be two or more sub-targets (e.g., three or more, fouror more, etc.) of the extended operating range metrology target, whereinthe sub-targets are configured to share a common center of symmetry andeach sub-target is invariant to 180 degree or more rotation about thecommon center of symmetry. Further, each sub-target may include two ormore periodic structures (e.g., three or more, four or more, etc.),wherein each of the periodic structures has an individual center ofsymmetry and each periodic structure is invariant to 180 degree or morerotation about the individual center of symmetry.

But, in an embodiment, the periodic structures of the sub-targets of anextended operating range metrology target may be rotationallyasymmetric. This may be accomplished in any of a number of ways. Forexample, a sub-target of three of more sub-targets may be shifted(located) away from a common center of symmetry of the othersub-targets. As another example, one or more of the features of one ormore of the periodic structures of a sub-target may be slightlyshortened, lengthened or shifted relative to one or more of the featuresof one or more other periodic structures of the sub-target or relativeto one or more of the features of one or more periodic structures ofanother sub-target. As another example, one or more dummy structures maybe inserted between periodic structures of a sub-target or betweensub-targets to disrupt any symmetry. In an embodiment, the one or moredummy structures are rotationally asymmetric. The shift, shortening orlengthening may be below the measurable range of the measurementapparatus. In an embodiment, the shift, shortening or lengthening is inthe 1 nm range or less. Such a change will have small to negligibleeffect on measurement readings. Similarly, the dummy structures may havefeature size or pitch below the effective measurement range of themeasurement apparatus. In an embodiment, the assist feature describedherein may be such a dummy structure.

The term “structure” is used herein without limitation to any particularform of structure such as a simple grating line. Indeed, coarsestructural features, such as the lines and spaces of a grating, can beformed by collections of finer sub-structures.

In association with the physical periodic structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a method of designing a target for a substrate,a method of producing a target on a substrate, a method of measuring atarget on a substrate and/or a method of analyzing a measurement toobtain information about a lithographic process. An embodiment maycomprise computer code containing one or more sequences ofmachine-readable instructions or data describing the target. Thiscomputer program or code may be executed for example within unit PU inthe apparatus of FIG. 3 and/or the control unit LACU of FIG. 2. Theremay also be provided a data storage medium (e.g., semiconductor memory,magnetic or optical disk, etc.) having such a computer program or codestored therein. Where an existing metrology apparatus, for example ofthe type shown in FIG. 3, is already in production and/or in use, anembodiment of the invention can be implemented by the provision of anupdated computer program product for causing a processor to perform oneor more of the method described herein. The computer program or code mayoptionally be arranged to control the optical system, substrate supportand the like to perform a method of measuring a parameter of thelithographic process on a suitable plurality of targets. The computerprogram or code can update the lithographic and/or metrology recipe formeasurement of further substrates. The computer program or code may bearranged to control (directly or indirectly) the lithographic apparatusfor the patterning and processing of further substrates.

Further embodiments according to the invention can be found in belownumbered clauses:

-   1. A method of measuring a parameter of a lithographic process, the    method comprising:

illuminating a diffraction measurement target on a substrate withradiation, the measurement target comprising at least a first sub-targetand at least a second sub-target, wherein the first and secondsub-targets each comprise a pair of periodic structures and wherein thefirst sub-target has a different design than the second sub-target, thedifferent design comprising the first sub-target periodic structureshaving a different pitch, feature width, space width, and/orsegmentation than the second sub-target periodic structures; and

detecting radiation scattered by at least the first and secondsub-targets to obtain for that target a measurement representing theparameter of the lithographic process.

-   2. The method of clause 1, wherein the first sub-target at least    partly overlays a fifth periodic structure and the second sub-target    at least partly overlays a sixth periodic structure, wherein the    fifth periodic structure is at a different layer on the substrate    than the sixth periodic structure.-   3. The method of clause 1 or clause 2, wherein features of the pair    of periodic structures of each of the first and second sub-targets    extend in a same direction.-   4. The method of clause 1 or clause 2, wherein the first and second    sub-targets each comprise a first pair of periodic structures having    features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction.-   5. The method of clause 2, wherein the first and second sub-targets    each comprise a first pair of periodic structures having features    extending in a first direction and a second pair of periodic    structures having features extending in a second different    direction, wherein the fifth periodic structure has features    extending in the first direction and the sixth periodic structure    has features extending in the second direction.-   6. The method of any of clauses 1-5, further comprising at least a    third sub-target and at least a fourth sub-target, wherein the third    and fourth sub-targets each comprise a pair of periodic structures.-   7. The method of clause 6, wherein the third sub-target at least    partly overlays a ninth periodic structure and the fourth sub-target    at least partly overlays a tenth periodic structure, wherein the    ninth periodic structure is at a different layer on the substrate    than the tenth periodic structure.-   8. The method of clause 6 or clause 7, wherein the third and fourth    sub-targets each comprise a first pair of periodic structures having    features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction.-   9. The method of clause 7, wherein the third and fourth sub-targets    each comprise a first pair of periodic structures having features    extending in a first direction and a second pair of periodic    structures having features extending in a second different    direction, wherein the ninth periodic structure has features    extending in the first direction and the tenth periodic structure    has features extending in the second direction.-   10. The method of any of clauses 1-9, wherein the parameter of the    lithographic process comprises overlay.-   11. The method of any of clauses 1-10, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    the periodic structures of the first and second sub-targets.-   12. The method of any of clauses 1-11, wherein at least part of each    of the periodic structures of the first and second sub-targets is    within a contiguous area of less than or equal to 1000 μm² on the    substrate.-   13. The method of any of clauses 1-11, wherein at least part of each    of the periodic structures of the first and second sub-targets is    within a contiguous area of less than or equal to 400 μm² on the    substrate.-   14. The method of any of clauses 1-13, wherein each of the    sub-targets is designed for a different process stack for the    substrate.-   15. The method of any of clauses 1-14, wherein each of the    sub-targets is designed for a different layer-pair for multiple    layer overlay measurement.-   16. A diffraction measurement target comprising at least a first    sub-target and at least a second sub-target, wherein the first and    second sub-targets each comprise a pair of periodic structures and    wherein the first sub-target has a different design than the second    sub-target, the different design comprising the first sub-target    periodic structures having a different pitch, feature width, space    width, and/or segmentation than the second sub-target periodic    structures.-   17. The target of clause 16, wherein, when on a substrate, the first    sub-target at least partly overlays a fifth periodic structure and    the second sub-target at least partly overlays a sixth periodic    structure, and the fifth periodic structure is at a different layer    than the sixth periodic structure.-   18. The target of clause 16 or clause 17, wherein features of the    pair of periodic structures of each of the first and second    sub-targets extend in a same direction.-   19. The target of clause 16 or clause 17, wherein the first and    second sub-targets each comprise a first pair of periodic structures    having features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction.-   20. The target of clause 17, wherein the first and second    sub-targets each comprise a first pair of periodic structures having    features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction, wherein the fifth periodic structure has features    extending in the first direction and the sixth periodic structure    has features extending in the second direction.-   21. The target of any of clauses 16-20, further comprising at least    a third sub-target and at least a fourth sub-target, wherein the    third and fourth sub-targets each comprise a pair of periodic    structures.-   22. The target of clause 21, wherein, when on a substrate, the third    sub-target at least partly overlays a ninth periodic structure and    the fourth sub-target at least partly overlays a tenth periodic    structure, and the ninth periodic structure is at a different layer    than the tenth periodic structure.-   23. The target of clause 21 or clause 22, wherein the third and    fourth sub-targets each comprise a first pair of periodic structures    having features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction.-   24. The target of clause 22, wherein the third and fourth    sub-targets each comprise a first pair of periodic structures having    features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction, wherein the ninth periodic structure has features    extending in the first direction and the tenth periodic structure    has features extending in the second direction.-   25. The target of any of clauses 16-24, wherein, when on a    substrate, at least part of each of the periodic structures of the    first and second sub-targets is within a contiguous area of less    than or equal to 1000 μm² on the substrate.-   26. The target of any of clauses 16-24, wherein, when on a    substrate, at least part of each of the periodic structures of the    first and second sub-targets is within a contiguous area of less    than or equal to 400 μm² on the substrate.-   27. A method of measuring a parameter of a lithographic process, the    method comprising:

illuminating a diffraction measurement target on a substrate withradiation, the measurement target comprising at least a first sub-targetand at least a second sub-target in a first layer, wherein the firstsub-target comprises a first periodic structure and the secondsub-target comprises a second periodic structure, wherein a thirdperiodic structure is located at least partly underneath the firstperiodic structure in a second different layer under the first layer andthere being no periodic structure underneath the second periodicstructure in the second layer, and wherein a fourth periodic structureis located at least partly underneath the second periodic structure in athird different layer under the second layer; and

detecting radiation scattered by at least the first through fourthperiodic structures to obtain for that target a measurement representingthe parameter of the lithographic process.

-   28. The method of clause 27, wherein the first sub-target has a    different design than the second sub-target.-   29. The method of clause 28, wherein the different design comprises    the first sub-target having a different pitch, feature width, space    width, and/or segmentation than the second sub-target.-   30. The method of any of clauses 27-29, wherein the first and second    sub-targets each comprise a further periodic structure having    features extending in a second direction different to a first    direction in which the features of the first and second periodic    structures respectively extend.-   31. The method of any of clauses 27-29, wherein the second periodic    structure has features extending in a different second direction to    a first direction in which features of the first periodic structure    extends.-   32. The method of clause 30 or clause 31, wherein the third periodic    structure has features extending in the first direction and the    fourth periodic structure has features extending in the second    direction.-   33. The method of any of clauses 27-29, wherein features of the    periodic structures of each of the first and second sub-targets and    of the third and fourth periodic structures extend in a same    direction.-   34. The method of any of clauses 27-33, further comprising at least    a third sub-target and at least a fourth sub-target, wherein the    third and fourth sub-targets each comprise a periodic structure.-   35. The method of clause 34, wherein the third sub-target at least    partly overlays a fifth periodic structure on the substrate and the    fourth sub-target at least partly overlays a sixth periodic    structure on the substrate, wherein the fifth periodic structure is    at a different layer than the sixth periodic structure.-   36. The method of clause 34 or clause 35, wherein the third    sub-target comprises a periodic structure having features extending    in a first direction and the fourth sub-target comprises a periodic    structure having features extending in a second different direction.-   37. The method of clause 35, wherein the third sub-target comprises    a periodic structure having features extending in a first direction    and the fourth sub-target comprises a periodic structure having    features extending in a second different direction, wherein the    fifth periodic structure has features extending in the first    direction and the sixth periodic structure has features extending in    the second direction.-   38. The method of any of clauses 27-37, wherein the parameter of the    lithographic process comprises overlay.-   39. The method of any of clauses 27-38, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    the periodic structures of the first and second sub-targets.-   40. The method of any of clauses 27-39, wherein at least part of    each of the periodic structures of the first and second sub-targets    is within a contiguous area of less than or equal to 1000 μm² on the    substrate.-   41. The method of any of clauses 27-39, wherein at least part of    each of the periodic structures of the first and second sub-targets    is within a contiguous area of less than or equal to 400 μm² on the    substrate.-   42. The method of any of clauses 27-41, wherein each of the    sub-targets is designed for a different process stack for the    substrate.-   43. The method of any of clauses 27-42, wherein each of the    sub-targets is designed for a different layer-pair for multiple    layer overlay measurement.-   44. A diffraction measurement target comprising at least a first    sub-target and at least a second sub-target that, when on a    substrate, are in a first layer, wherein the first sub-target    comprises a first periodic structure and the second sub-target    comprises a second periodic structure, and comprising a third    periodic structure, when on the substrate, located at least partly    underneath the first periodic structure in a second different layer    under the first layer and there being no periodic structure    underneath the second periodic structure in the second layer, and    comprising a fourth periodic structure, when on the substrate,    located at least partly underneath the second periodic structure in    a third different layer under the second layer.-   45. The target of clause 44, wherein the first sub-target has a    different design than the second sub-target.-   46. The target of clause 45, wherein the different design comprises    the first sub-target having a different pitch, feature width, space    width, and/or segmentation than the second sub-target.-   47. The target of any of clauses 44-46, wherein the first and second    sub-targets each comprise a further periodic structure having    features extending in a second direction different to a first    direction in which the features of the first and second periodic    structures respectively extend.-   48. The target of any of clauses 44-47, wherein the second periodic    structure has features extending in a different second direction to    a first direction in which features of the first periodic structure    extends.-   49. The target of clause 47 or clause 48, wherein the third periodic    structure has features extending in the first direction and the    fourth periodic structure has features extending in the second    direction.-   50. The target of any of clauses 44-46, wherein features of the    periodic structures of each of the first and second sub-targets and    of the third and fourth periodic structures extend in a same    direction.-   51. The target of any of clauses 46-50, further comprising at least    a third sub-target and at least a fourth sub-target, wherein the    third and fourth sub-targets each comprise a periodic structure.-   52. The target of clause 51, wherein, when on a substrate, the third    sub-target at least partly overlays a fifth periodic structure on    the substrate and the fourth sub-target at least partly overlays a    sixth periodic structure on the substrate, and the fifth periodic    structure is at a different layer than the sixth periodic structure.-   53. The target of clause 51 or clause 52, wherein the third    sub-target comprises a periodic structure having features extending    in a first direction and the fourth sub-target comprises a periodic    structure having features extending in a second different direction.-   54. The target of clause 52, wherein the third sub-target comprises    a periodic structure having features extending in a first direction    and the fourth sub-target comprises a periodic structure having    features extending in a second different direction, wherein the    fifth periodic structure has features extending in the first    direction and the sixth periodic structure has features extending in    the second direction.-   55. The target of any of clauses 44-54, wherein, when on the    substrate, at least part of each of the periodic structures of the    first and second sub-targets is within a contiguous area of less    than or equal to 1000 μm².-   56. The target of any of clauses 44-55, wherein, when on the    substrate, at least part of each of the periodic structures of the    first and second sub-targets is within a contiguous area of less    than or equal to 400 μm².-   57. A method of measuring a parameter of a lithographic process, the    method comprising:

illuminating a diffraction measurement target on a substrate withradiation, the measurement target comprising at least a first sub-targetand at least a second sub-target, wherein the first and secondsub-targets each comprise a first pair of periodic structures havingfeatures extending in a first direction and a second pair of periodicstructures having features extending in a second different direction,and wherein the first sub-target has a different design than the secondsub-target; and

detecting radiation scattered by at least the first and secondsub-targets to obtain for that target a measurement representing theparameter of the lithographic process.

-   58. The method of clause 57, wherein at least one of the periodic    structures of the first sub-target has a first period and a first    feature or space width, wherein at least one of the periodic    structures of the second sub-target has a second period and a second    feature or space width, and wherein the different design comprises    the first period, the first feature or space width, or both, being    different than respectively the second period, the second feature or    space width, or both.-   59. The method of clause 57 or clause 58, wherein the first    sub-target at least partly overlays a ninth periodic structure and    the second sub-target at least partly overlays a tenth periodic    structure, wherein the ninth periodic structure is at a different    layer on the substrate than the tenth periodic structure.-   60. The method of clause 59, wherein features of the ninth periodic    structure extend in the first direction and features of the tenth    periodic structure extend in the second direction.-   61. The method of clause 60, further comprising at least a third    sub-target and at least a fourth sub-target, wherein the third and    fourth sub-targets each comprise a periodic structure.-   62. The method of clause 61, wherein the third sub-target at least    partly overlays a thirteenth periodic structure and the fourth    sub-target at least partly overlays a fourteenth periodic structure,    wherein the thirteenth periodic structure is at a different layer on    the substrate than the fourteenth periodic structure and thirteenth    and fourteenth periodic structures are at different layers than the    ninth and tenth periodic structures.-   63. The method of any of clauses 57-62, wherein the measurement    target fits within an area of 400 μm².-   64. The method of any of clauses 57-63, wherein the parameter of the    lithographic process comprises overlay.-   65. The method of any of clauses 57-64, wherein each of the    sub-targets is designed for a different process stack for the    substrate.-   66. The method of any of clauses 57-65, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    the periodic structures of the first and second sub-targets.-   67. A diffraction measurement target comprising at least a first    sub-target and at least a second sub-target, wherein the first and    second sub-targets each comprise a first pair of periodic structures    having features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction, and wherein the first sub-target has a different design    than the second sub-target.-   68. The target of clause 67, wherein, when on a substrate, the first    sub-target at least partly overlays a ninth periodic structure and    the second sub-target at least partly overlays a tenth periodic    structure, and the ninth periodic structure is at a different layer    on the substrate than the tenth periodic structure.-   69. The target of clause 68, wherein features of the ninth periodic    structure extend in the first direction and features of the tenth    periodic structure extend in the second direction.-   70. The target of clause 69, further comprising at least a third    sub-target and at least a fourth sub-target, wherein the third and    fourth sub-targets each comprise a periodic structure.-   72. The target of clause 70, wherein, when on a substrate, the third    sub-target at least partly overlays a thirteenth periodic structure    and the fourth sub-target at least partly overlays a fourteenth    periodic structure, wherein the thirteenth periodic structure is at    a different layer on the substrate than the fourteenth periodic    structure and thirteenth and fourteenth periodic structures are at    different layers than the ninth and tenth periodic structures.-   73. The target of any of clauses 67-72, wherein, when on a    substrate, the measurement target fits within an area of 400 μm².-   74. The target of any of clauses 67-73, wherein at least one of the    periodic structures of the first sub-target has a first period and a    first feature or space width, wherein at least one of the periodic    structures of the second sub-target has a second period and a second    feature or space width, and wherein the different design comprises    the first period, the first feature or space width, or both, being    different than respectively the second period, the second feature or    space width, or both.-   75. The target of any of clauses 67-74, wherein each of the    sub-targets is designed for a different process stack for a    lithographic substrate.-   76. A method of measuring a parameter of a lithographic process, the    method comprising:

illuminating a diffraction measurement target on a substrate withradiation, the measurement target comprising at least a first sub-targetand at least a second sub-target, wherein the first and secondsub-targets each comprise a first pair of periodic structures havingfeatures extending in a first direction and a second pair of periodicstructures having features extending in a second different direction,and wherein at least part of each of the periodic structures of thefirst and second sub-targets is within a contiguous area of less than orequal to 1000 μm² on the substrate; and

detecting radiation scattered by at least the first and secondsub-targets to obtain for that target a measurement representing theparameter of the lithographic process.

-   77. The method of clause 76, wherein the first sub-target has a    different design than the second sub-target.-   78. The method of clause 77, wherein the different design comprises    the first sub-target having a different pitch, feature width, space    width, and/or segmentation than the second sub-target.-   79. The method of any of clauses 76-78, wherein the first sub-target    at least partly overlays a ninth periodic structure and the second    sub-target at least partly overlays a tenth periodic structure,    wherein the ninth periodic structure is at a different layer on the    substrate than the tenth periodic structure.-   80. The method of clause 79, wherein features of the ninth periodic    structure extend in the first direction and features of the tenth    periodic structure extend in the second direction.-   81. The method of clause 80, further comprising at least a third    sub-target and at least a fourth sub-target, wherein the third and    fourth sub-targets each comprise a periodic structure.-   82. The method of clause 81, wherein the third sub-target at least    partly overlays a thirteenth periodic structure and the fourth    sub-target at least partly overlays a fourteenth periodic structure,    wherein the thirteenth periodic structure is at a different layer on    the substrate than the fourteenth periodic structure and thirteenth    and fourteenth periodic structures are at different layers than the    ninth and tenth periodic structures.-   83. The method of any of clauses 76-82, wherein the parameter of the    lithographic process comprises overlay.-   84. The method of any of clauses 76-83, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    the periodic structures of the first and second sub-targets.-   85. The method of any of clauses 76-84, wherein at least part of    each of the periodic structures of the first and second sub-targets    is within a contiguous area of less than or equal to 400 μm² on the    substrate.-   86. The method of any of clauses 76-85, wherein each of the    sub-targets is designed for a different process stack for the    substrate.-   87. The method of any of clauses 76-86, wherein each of the    sub-targets is designed for a different layer-pair for multiple    layer overlay measurement.-   88. A diffraction measurement target comprising at least a first    sub-target and at least a second sub-target, wherein the first and    second sub-targets each comprise a first pair of periodic structures    having features extending in a first direction and a second pair of    periodic structures having features extending in a second different    direction, and wherein at least part of each of the periodic    structures of the first and second sub-targets is within a    contiguous area of less than or equal to 1000 μm² on a substrate.-   89. The target of clause 88, wherein the second sub-target has a    different design than the first-sub target.-   90. The target of clause 88 or clause 89, wherein, when on a    substrate, the first sub-target at least partly overlays a ninth    periodic structure and the second sub-target at least partly    overlays a tenth periodic structure, and the ninth periodic    structure is at a different layer on the substrate than the tenth    periodic structure.-   91. The target of clause 90, wherein features of the ninth periodic    structure extend in the first direction and features of the tenth    periodic structure extend in the second direction.-   92. The target of clause 91, further comprising at least a third    sub-target and at least a fourth sub-target, wherein the third and    fourth sub-targets each comprise a periodic structure.-   93. The target of clause 92, wherein, when on a substrate, the third    sub-target at least partly overlays a thirteenth periodic structure    and the fourth sub-target at least partly overlays a fourteenth    periodic structure, wherein the thirteenth periodic structure is at    a different layer on the substrate than the fourteenth periodic    structure and thirteenth and fourteenth periodic structures are at    different layers than the ninth and tenth periodic structures.-   94. The target of any of clauses 88-93, wherein, when on the    substrate, at least part of each of the periodic structures of the    first and second sub-targets is within a contiguous area of less    than or equal to 400 μm².-   95. A method of metrology target design, the method comprising:

receiving an indication for the design of a diffractive metrology targethaving a plurality of sub-targets, each sub-target comprising a firstpair of periodic structures having features extending in a firstdirection and a second pair of periodic structures having featuresextending in a second different direction;

receiving a constraint on the area, a dimension, or both, of thediffractive metrology target; and

selecting, by a processor, a design of the diffractive metrology targetbased at least on the constraint.

-   96. The method of clause 95, wherein the constraint on the area, a    dimension, or both, of the diffractive metrology target comprises at    least part of each of the periodic structures of the first and    second sub-targets is within a contiguous area of less than or equal    to 1000 μm² on the substrate.-   97. The method of clause 95 or clause 96, further comprising    receiving information regarding at least two different process    stacks and wherein the design of the diffractive metrology target    comprises each of the sub-targets being designed for a different    process stack.-   98. The method of any of clauses 95-97, further comprising receiving    information regarding multiple layer-pairs to be measured by the    diffractive metrology target and wherein the design of the    diffractive metrology target comprises each of the sub-targets being    designed for a different layer-pair.-   99. The method of any of clauses 95-98, wherein the design of the    diffractive metrology target comprises the first sub-target having a    different pitch, feature width, space width, and/or segmentation    than the second sub-target.-   100. A method comprising:

illuminating with radiation a diffraction measurement target on asubstrate, the measurement target comprising at least a firstsub-target, a second sub-target and a third sub-target, wherein thefirst, second and third sub-targets are different in design.

-   101. The method of clause 100, wherein the different design    comprises one of the first to third sub-targets having a different    pitch, feature width, space width, and/or segmentation than another    of the first to third sub-targets.-   102. The method of clause 100 or clause 101, wherein the first    sub-target at least partly overlays a first periodic structure, the    second sub-target at least partly overlays a second periodic    structure, and the second sub-target at least partly overlays a    second periodic structure, wherein the first periodic structure is    at a different layer on the substrate than the second and third    periodic structures and the second periodic structure is at a    different layer on the substrate than the first and second periodic    structures.-   103. The method of any of clauses 100-102, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    a periodic structure of the first to third sub-targets.-   104. The method of any of clauses 100-103, wherein at least part of    a periodic structure of each of the first to third sub-targets is    within a contiguous area of less than or equal to 400 μm² on the    substrate.-   105. The method of any of clauses 100-104, wherein each of the first    to third sub-targets is designed for a different process stack for    the substrate.-   106. The method of any of clauses 100-105, wherein each of the first    to third sub-targets is designed for a different layer-pair for    multiple layer overlay measurement.-   107. A diffraction metrology target comprising at least a first    sub-target, a second sub-target and a third sub-target, wherein the    first, second and third sub-targets are different in design.-   108. The target of clause 107, wherein the different design    comprises one of the first to third sub-targets having a different    pitch, feature width, space width, and/or segmentation than another    of the first to third sub-targets.-   109. The target of clause 107 or clause 108, wherein at least part    of a periodic structure of each of the first to third sub-targets is    within a contiguous area of less than or equal to 400 μm².-   110. The target of any of clauses 107-109, wherein each of the first    to third sub-targets is designed for a different process stack for a    substrate.-   111. The target of any of clauses 107-110, wherein each of the first    to third sub-targets is designed for a different layer-pair for    multiple layer overlay measurement.-   112. A method comprising measuring overlay between two layers, the    method comprising:

illuminating with radiation a diffraction measurement target on asubstrate having a portion of the target on each of the two layers,wherein the two layers are separated by at least one other layer.

-   113. The method of clause 112, wherein a first layer of the two    layers comprises at least a first sub-target and a second    sub-target, wherein a first periodic structure is located at least    partly underneath the first sub-target in a second layer of the two    layers and there being no periodic structure underneath the second    sub-target in the second layer.-   114. The method of clause 113, wherein the first and second    sub-targets are different in design.-   115. The method of clause 114, wherein the different design    comprises the first sub-target having a different pitch, feature    width, space width, and/or segmentation than the second sub-target.-   116. The method of any of clauses 113-115, wherein a second periodic    structure is located at least partly underneath the second    sub-target in the at least one other layer.-   117. The method of any of clauses 113-116, wherein illuminating    comprising illuminating a measurement spot on the diffraction    measurement target that covers at one time at least part of each of    a periodic structure of the first and second sub-targets.-   118. The method of any of clauses 113-117, wherein at least part of    a periodic structure of each of the first and second sub-targets is    within a contiguous area of less than or equal to 400 μm² on the    substrate.-   119. The method of any of clauses 113-118, wherein each of the first    and second sub-targets is designed for a different process stack for    the substrate.-   120. The method of any of clauses 113-119, wherein each of the first    and second sub-targets is designed for a different layer-pair for    multiple layer overlay measurement.-   121. A method of devising an arrangement of a diffraction    measurement target, the target comprising a plurality of    sub-targets, each sub-target comprising a plurality of periodic    structures and each sub-target designed to measure a different    layer-pair or to measure for a different process stack, the method    comprising:

locating the periodic structures of the sub-targets within a targetarea; and

locating an assist feature at a periphery of at least one of thesub-targets, the assist feature configured to reduce a measuredintensity peak at the periphery of the at least one sub-target.

-   122. The method of clause 121, wherein the assist feature adjacent    to and orientated with a particular periodic structure of a    sub-target is positioned in phase with that periodic structure.-   123. The method of clause 121 or clause 122, wherein the assist    feature comprises a plurality of assist features and the target area    is defined by the plurality of assist features substantially    surrounding the target area.-   124. The method of clause 123, wherein the assist feature comprises    a further plurality of assist features provided between each    sub-target within the target area.-   125. The method of clause 124, wherein the further plurality of    assist features are located to fill a space between the sub-targets    comprising at least a half a wavelength of the relevant inspection    wavelength.-   126. The method of any of clauses 121-125, wherein each sub-target    is substantially surrounded by the assist feature so as to isolate    each sub-target from its surrounding environment.-   127. The method of any of clauses 121-126, wherein the assist    feature comprises features having a pitch substantially smaller than    a pitch of a periodic structure of a sub-target of the plurality of    sub-targets.-   128. The method of any of clauses 121-127, wherein a pitch of a    plurality of structures of the assist feature is such that the    assist feature is not detected during inspection of the target using    a metrology process.-   129. The method of any of clauses 121-128, wherein the assist    feature is located immediately adjacent each outermost substructure    of each sub-target.-   130. The method of any of clauses 121-129, further comprising:

modelling a resultant image obtained by inspection of the target using adiffraction-based metrology process; and

evaluating whether the target arrangement is optimized for detectionusing the diffraction-based metrology process.

-   131. The method of clause 130, wherein the method is repeated    iteratively in order to optimize the target arrangement.-   132. The method of clause 130 or clause 131, wherein a criteria for    considering whether a particular target arrangement is considered    optimized include at least one selected from:

determining whether intensities at the sub-target periphery are of thesame order of magnitude as those at the sub-target center, wheninspected using the diffraction-based metrology process;

determining whether there is minimum intensity variation at thesub-target periphery in the presence of overlay, defocus and/oraberrations when inspected using the diffraction-based metrologyprocess;

determining whether there is sufficient spacing between the sub-targetsfor optimum target-recognition for the relevant inspection wavelengthrange; and/or

determining whether the total grating area is maximized.

-   133. The method of any of clauses 121-132, wherein the target    comprises two or more overlaid target layers, with an upper target    layer comprising an overlay bias and the assist feature, and wherein    the bias is not applied to the assist feature in the upper layer.-   134. The method of any of clauses 121-132, wherein the target    comprises two or more overlaid target layers, with an upper target    layer comprising an overlay bias, and wherein the upper layer does    not comprise any assist feature.-   135. The method of any of clauses 121-132, wherein the assist    feature is located in a different layer than the at least one    sub-target.-   136. A diffraction measurement target comprising:

a plurality of sub-targets in a target area of the target, eachsub-target comprising a plurality of periodic structures and eachsub-target designed to measure a different layer-pair or to measure fora different process stack; and

an assist feature at the periphery of at least one of the sub-targets,the assist feature configured to reduce a measured intensity peak at theperiphery of the sub-targets.

-   137. The target of clause 136, wherein the assist feature comprises    features having a pitch substantially smaller than a pitch of a    periodic structure of a sub-target of the plurality of sub-targets.-   138. The target of clause 136 or clause 137, wherein each sub-target    is substantially surrounded by the assist feature so as to isolate    each sub-target from its surrounding environment.-   139. The target of any of clauses 136-138, wherein the assist    feature comprises a plurality of assist features and the plurality    of assist features substantially surround the target area.-   140. The target of clause 139, wherein the assist feature comprises    a further plurality of assist features which are provided between    each sub-target within the target area.-   141. The target of any of clauses 136-140, wherein a pitch of    features of the assist feature is such that the assist feature is    not detected during inspection of the target using a metrology    process.-   142. The target of any of clauses 136-141, wherein the assist    feature is configured to reduce diffraction intensity peaks at each    sub-target periphery.-   143. The target of any of clauses 136-142, wherein the assist    feature is located immediately adjacent each outermost substructure    of each sub-target.-   144. The target of any of clauses 136-143, wherein the assist    feature adjacent to and orientated with a particular periodic    structure of a sub-target is positioned in phase with that periodic    structure.-   145. A method of manufacturing devices wherein a device pattern is    applied to a series of substrates using a lithographic process, the    method including inspecting at least a diffraction measurement    target formed as part of or beside the device pattern on at least    one of the substrates using the method of any of clauses 1-9, 15-24,    31-37, 43-51, 61-67, or 73-81, and controlling the lithographic    process for later substrates in accordance with the result of the    method.-   146. A non-transitory computer program product comprising    machine-readable instructions for causing a processor to cause    performance of the method of any of clauses 1-9, 15-24, 31-37,    43-51, 56-67, 73-81 or 82-96.-   147. A non-transitory computer program product comprising    machine-readable instructions or data defining the target of any of    clauses 10-14, 25-30, 38-42, 52-55, 68-72 or 97-105.-   148. A substrate comprising the target of any of clauses 10-14,    25-30, 38-42, 52-55, 68-72 or 97-105.-   149. A patterning device configured to at least in part form the    diffraction measurement target according to any of clauses 10-14,    25-30, 38-42, 52-55, 68-72 or 97-105.-   150. A system comprising:

an inspection apparatus configured to provide a beam on a diffractionmeasurement target on a substrate and to detect radiation diffracted bythe target to determine a parameter of a lithographic process; and

the non-transitory computer program product of clause 146 or clause 147.

-   151. The system of clause 150, further comprising a lithographic    apparatus comprising a support structure configured to hold a    patterning device to modulate a radiation beam and a projection    optical system arranged to project the modulated onto a    radiation-sensitive substrate.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments reveals thegeneral nature of embodiments of the invention such that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt for various applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1.-12. (canceled)
 13. A method comprising measuring overlay between twolayers, the method comprising: illuminating with radiation a diffractionmeasurement target on a substrate having a portion of the target on eachof the two layers, wherein the two layers are separated by at least oneother layer.
 14. The method of claim 13, wherein a first layer of thetwo layers comprises at least a first sub-target and a secondsub-target, wherein a first periodic structure is located at leastpartly underneath the first sub-target in a second layer of the twolayers and there being no periodic structure underneath the secondsub-target in the second layer.
 15. The method of claim 14, wherein thefirst and second sub-targets are different in design.
 16. The method ofclaim 15, wherein the different design comprises the first sub-targethaving a different pitch, feature width, space width, and/orsegmentation than the second sub-target.
 17. The method of claim 14,wherein a second periodic structure is located at least partlyunderneath the second sub-target in the at least one other layer. 18.The method of claim 14, wherein illuminating comprising illuminating ameasurement spot on the diffraction measurement target that covers atone time at least part of each of a periodic structure of the first andsecond sub-targets.
 19. The method of claim 14, wherein at least part ofa periodic structure of each of the first and second sub-targets iswithin a contiguous area of less than or equal to 400 μm² on thesubstrate.
 20. The method of claim 14, wherein each of the first andsecond sub-targets is designed for a different process stack for thesubstrate.
 21. The method of claim 14, wherein each of the first andsecond sub-targets is designed for a different layer-pair for multiplelayer overlay measurement.
 22. A method of manufacturing devices whereina device pattern is applied to a series of substrates using alithographic process, the method including inspecting at least adiffraction measurement target formed as part of or beside the devicepattern on at least one of the substrates using the method of claim 13to obtain a result, and controlling the lithographic process for one ormore substrates in accordance with the result.
 23. A non-transitorycomputer program product comprising machine-readable instructionstherein, the instructions, upon execution by a computer system,configured to cause the computer system to at least: determine overlaybetween two layers by causing illumination with radiation of adiffraction measurement target on a substrate having a portion of thetarget on each of the two layers, wherein the two layers are separatedby at least one other layer.
 24. The computer program product of claim23, wherein a first layer of the two layers comprises at least a firstsub-target and a second sub-target, wherein a first periodic structureis located at least partly underneath the first sub-target in a secondlayer of the two layers and there being no periodic structure underneaththe second sub-target in the second layer.
 25. The method of claim 24,wherein the first and second sub-targets are different in design. 26.The method of claim 25, wherein the different design comprises the firstsub-target having a different pitch, feature width, space width, and/orsegmentation than the second sub-target.
 27. A system comprising: aninspection apparatus configured to provide a beam on a diffractionmeasurement target on a substrate and to detect radiation diffracted bythe target to determine a parameter of a lithographic process; and thenon-transitory computer program product of claim
 23. 28. A diffractionmeasurement target for determination of overlay between two layers, thetarget comprising: a first portion of the target configured for a firstlayer of the two layers, a second portion of the target configured for asecond layer of the two layers, wherein the two layers are separated byat least one other layer.
 29. The target of claim 28, wherein the firstportion comprises at least a first sub-target and a second sub-target,wherein a first periodic structure, when on a substrate, is located atleast partly underneath the first sub-target in the second layer andthere being no periodic structure on the substrate underneath the secondsub-target in the second layer.
 30. The target of claim 29, wherein thefirst and second sub-targets are different in design.
 31. The target ofclaim 28, wherein at least part of a periodic structure of each of thefirst and second portions of the target is within a contiguous area ofless than or equal to 400 μm² on a substrate having the target.
 32. Asubstrate comprising the target of claim 28.